feat(implement-issue): make child agent dispatch agent-agnostic

- Add formal arg syntax: --agent, --base, --force
- Add process-tree detection scripts (Linux / macOS) to auto-detect parent agent
- Add dispatch table supporting pi, opencode, goose, codex, claude + unknown fallback
- Write child prompt to temp file instead of inline escaping
- Name tmux windows for discoverability
- Add new pre-flight checks: tmux session, agent detection, binary on PATH

common/engineering/implement-issue/SKILL.md

@@ -1,5 +1,6 @@
 ---
 name: implement-issue
+description: Dispatch a child agent in an isolated git worktree to implement a `ready-for-agent` issue end-to-end.
 disable-model-invocation: true
 ---
 
@@ -9,6 +10,17 @@ Dispatch a child agent in an isolated git worktree to implement a `ready-for-age
 
 The issue tracker conventions live in [`docs/agents/issue-tracker.md`](../../../docs/agents/issue-tracker.md) and the triage label vocabulary in [`docs/agents/triage-labels.md`](../../../docs/agents/triage-labels.md). Both should have been provided to you already.
 
+## Invocation
+
+```
+/skill:implement-issue <N> [--agent <name>] [--base <branch>] [--force]
+```
+
+- `<N>` — required, the issue number
+- `--agent <name>` — optional, one of `pi`, `opencode`, `goose`, `codex`, `claude`. If omitted, auto-detected from the process tree.
+- `--base <branch>` — optional, target base branch (default: repo default branch).
+- `--force` — optional, allow overwriting an existing worktree.
+
 ## Process
 
 ### 1. Pre-flight checks
@@ -79,6 +91,37 @@ ls -d ../<repo-name>-issue-<N>
 
 If it exists, report and stop. Let the user override with `--force`.
 
+#### 1h. Running inside a tmux session
+
+Confirm `$TMUX` is set. If not, report "This skill requires a tmux session. Start tmux and rerun." and stop.
+
+#### 1i. Detect the agent binary
+
+If `--agent <name>` was provided in the invocation args, use that name directly. Otherwise, run the platform-appropriate detection script:
+
+```bash
+bash ./detect-agent-linux.sh
+# or on macOS:
+bash ./detect-agent-macos.sh
+```
+
+The script outputs the agent name on success (exit 0), or nothing on failure (exit 1).
+
+If detection fails (no `--agent` flag and script returns nothing), report:
+
+> Could not detect which agent is running. Rerun with `--agent <name>`.
+> Supported agents: pi, opencode, goose, codex, claude.
+
+Stop.
+
+If detection succeeds, confirm the binary is on PATH:
+
+```bash
+command -v <agent>
+```
+
+If the binary is not found, report "Agent binary '<agent>' not found on PATH." and stop.
+
 ### 2. Setup
 
 #### 2a. Fetch latest base
@@ -99,7 +142,7 @@ tea issues edit <N> --add-labels "in-progress" --remove-labels "ready-for-agent"
 git worktree add -b <branch-name> ../<repo-name>-issue-<N> <base>
 ```
 
-### 3. Compose the child prompt
+### 3. Compose and write the child prompt
 
 Assemble a single prompt that the child agent will receive. Include:
 
@@ -120,15 +163,46 @@ Assemble a single prompt that the child agent will receive. Include:
   10. Print `DONE — issue #<N>`
 - **Failure instruction**: "If any step fails, report where you stopped and what remains for manual recovery. Print the exact commands needed."
 
-### 4. Launch the child
+Write the full prompt to a temp file:
-
-Open a new tmux window in the worktree directory and run `pi` with the composed prompt:
 
 ```bash
-tmux new-window -c <absolute-worktree-path> "pi --prompt '<escaped-prompt>'"
+cat > /tmp/issue-<N>-prompt.md <<'PROMPT_EOF'
+<full-composed-prompt>
+PROMPT_EOF
 ```
 
-The pane stays open after the child completes so the user can review the output.
+### 4. Launch the child
+
+Look up the full shell command from the dispatch table below, using the agent name detected in step 1i.
+
+| Agent | Shell command |
+|-------|--------------|
+| `pi` | `pi -p @/tmp/issue-<N>-prompt.md` |
+| `opencode` | `opencode run -f /tmp/issue-<N>-prompt.md` |
+| `goose` | `goose run -i /tmp/issue-<N>-prompt.md` |
+| `codex` | `cat /tmp/issue-<N>-prompt.md \| codex exec` |
+| `claude` | `cat /tmp/issue-<N>-prompt.md \| claude -p` |
+| *unknown* | `cat /tmp/issue-<N>-prompt.md \| <binary>` |
+
+Substitute the actual temp file path and issue number, then launch in a new tmux window:
+
+```bash
+tmux new-window -n "issue-<N>-<repo>" -c <absolute-worktree-path> "<shell-command>"
+```
+
+For example, with pi:
+
+```bash
+tmux new-window -n "issue-42-myrepo" -c ../myrepo-issue-42 "pi -p @/tmp/issue-42-prompt.md"
+```
+
+The window stays open after the child completes so the user can review the output.
+
+After tmux launches, clean up the temp file:
+
+```bash
+rm /tmp/issue-<N>-prompt.md
+```
 
 ### 5. Print summary
 
@@ -137,7 +211,8 @@ After launching, print:
 - Worktree path
 - Branch name
 - Base branch
-- Tmux window (so the user can find it)
+- Agent used
+- Tmux window name (so the user can find it)
 - Cleanup command: `git worktree remove <worktree-path> && git worktree prune`
 
 The parent's turn ends here. The user can dispatch another issue immediately.

common/engineering/implement-issue/detect-agent-linux.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+# Walk the process tree upward from $PPID to find a known agent binary.
+# Outputs the agent name on stdout if found, exits 0.
+# Outputs nothing and exits 1 if no known agent is found.
+
+set -euo pipefail
+
+# Dispatch table: binary names we recognize.
+KNOWN_AGENTS="pi opencode goose codex claude"
+
+pid=$PPID
+
+while [ "$pid" -gt 1 ]; do
+    # Read command name and parent PID from /proc.
+    comm=$(cat /proc/$pid/comm 2>/dev/null) || { pid=1; continue; }
+    ppid=$(awk '{print $4}' /proc/$pid/stat 2>/dev/null) || ppid=1
+
+    # Direct match against known binaries.
+    for agent in $KNOWN_AGENTS; do
+        if [ "$comm" = "$agent" ]; then
+            echo "$agent"
+            exit 0
+        fi
+    done
+
+    # If the process is node or bun, inspect cmdline for agent script paths.
+    case "$comm" in
+        node|bun)
+            cmdline=$(tr '\0' ' ' < /proc/$pid/cmdline 2>/dev/null) || cmdline=""
+            for agent in $KNOWN_AGENTS; do
+                case "$cmdline" in
+                    *"$agent"*)
+                        echo "$agent"
+                        exit 0
+                        ;;
+                esac
+            done
+            ;;
+    esac
+
+    pid=$ppid
+done
+
+exit 1

common/engineering/implement-issue/detect-agent-macos.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+# Walk the process tree upward from $PPID to find a known agent binary.
+# Outputs the agent name on stdout if found, exits 0.
+# Outputs nothing and exits 1 if no known agent is found.
+#
+# macOS version — uses ps(1) since /proc is not available.
+# Requires bash 3.2+ (macOS ships bash 3.2).
+
+set -euo pipefail
+
+KNOWN_AGENTS="pi opencode goose codex claude"
+
+pid=$PPID
+
+while [ "$pid" -gt 1 ]; do
+    # Get parent PID and command name.
+    ppid=$(ps -o ppid= -p "$pid" 2>/dev/null | tr -d ' ') || ppid=1
+    comm=$(ps -o comm= -p "$pid" 2>/dev/null) || { pid=$ppid; continue; }
+    # Strip leading path, keep only the basename.
+    comm=$(basename "$comm" 2>/dev/null || echo "$comm")
+
+    # Direct match against known binaries.
+    for agent in $KNOWN_AGENTS; do
+        if [ "$comm" = "$agent" ]; then
+            echo "$agent"
+            exit 0
+        fi
+    done
+
+    # If the process is node or bun, inspect full command line for agent names.
+    case "$comm" in
+        node|bun)
+            args=$(ps -o args= -p "$pid" 2>/dev/null) || args=""
+            for agent in $KNOWN_AGENTS; do
+                case "$args" in
+                    *"$agent"*)
+                        echo "$agent"
+                        exit 0
+                        ;;
+                esac
+            done
+            ;;
+    esac
+
+    pid=$ppid
+done
+
+exit 1