feat(implement-issue): add Section E to setup-skills for agent CLI selection

- Add agents-seed.md as source of truth for supported agents
- Add Section E to setup-skills to present agent menu and validate binary on PATH
- Refactor implement-issue to read docs/agents/agent-cli.md instead of hardcoded table
- Remove detect-agent-linux.sh and detect-agent-macos.sh detection scripts
- Add mise//usr/bin/fish -c runtime detection for agent dispatch
- Add invocation-time binary validation with clear error message
- Add test file documenting expected tmux command composition

Closes #9

common/engineering/implement-issue/agent-cli.test.md

@@ -0,0 +1,161 @@
+# Agent CLI Tests for implement-issue
+
+These tests verify the tmux command composition logic in `implement-issue`. They intercept the generated shell command string rather than the tmux invocation itself.
+
+The seam being tested: given a `docs/agents/agent-cli.md` config and a `mise` availability state, `implement-issue` must compose the correct tmux launch command.
+
+## Test fixtures
+
+### Fixture: pi agent config (stdin-piped via @{prompt})
+
+```yaml
+# docs/agents/agent-cli.md
+selected: pi
+binary: pi
+args: "-p @{prompt}"
+```
+
+### Fixture: opencode agent config
+
+```yaml
+# docs/agents/agent-cli.md
+selected: opencode
+binary: opencode
+args: "run -f @{prompt}"
+```
+
+### Fixture: codex agent config (empty args = stdin pipe)
+
+```yaml
+# docs/agents/agent-cli.md
+selected: codex
+binary: codex
+args: ""
+```
+
+### Fixture: missing binary config
+
+```yaml
+# docs/agents/agent-cli.md
+selected: missing-agent
+binary: totally-not-on-path
+args: ""
+```
+
+## Test cases
+
+### Test: composes correct tmux command for pi with mise available
+
+**Given:**
+- `docs/agents/agent-cli.md` contains the pi fixture
+- `command -v mise` succeeds
+
+**When:** `implement-issue` composes the tmux launch command
+
+**Then:** the generated command string is:
+
+```
+tmux new-window -n "issue-42-ws-sjb-skills" -c /home/sjb/Projects/personal/ws-sjb-skills/ws-sjb-skills-issue-42 "mise x --allow-env='*' -- pi -p /tmp/issue-42-prompt.md"
+```
+
+**Notes:**
+- `@{prompt}` is substituted with the actual temp file path
+- Window name includes issue number and repo slug
+- Worktree path is absolute
+- `mise x --allow-env='*'` is prepended when mise is available
+
+---
+
+### Test: composes correct tmux command for pi without mise (fallback to $SHELL -c)
+
+**Given:**
+- `docs/agents/agent-cli.md` contains the pi fixture
+- `command -v mise` fails (no mise on PATH)
+
+**When:** `implement-issue` composes the tmux launch command
+
+**Then:** the generated command string is:
+
+```
+tmux new-window -n "issue-42-ws-sjb-skills" -c /home/sjb/Projects/personal/ws-sjb-skills/ws-sjb-skills-issue-42 "$SHELL -c 'pi -p /tmp/issue-42-prompt.md'"
+```
+
+**Notes:**
+- Falls back to `$SHELL -c` when mise is absent
+- The agent command is wrapped inside `$SHELL -c '...'`
+
+---
+
+### Test: composes correct tmux command for codex (stdin piping, no @{prompt})
+
+**Given:**
+- `docs/agents/agent-cli.md` contains the codex fixture (args: "")
+- `command -v mise` succeeds
+
+**When:** `implement-issue` composes the tmux launch command
+
+**Then:** the generated command string is:
+
+```
+tmux new-window -n "issue-42-ws-sjb-skills" -c /home/sjb/Projects/personal/ws-sjb-skills/ws-sjb-skills-issue-42 "mise x --allow-env='*' -- sh -c 'cat /tmp/issue-42-prompt.md | codex'"
+```
+
+**Notes:**
+- When args is empty, prompt is piped via `cat <prompt> | <binary>`
+- The pipe syntax is always `cat <prompt> | <binary>` for stdin-based agents
+
+---
+
+### Test: fails fast with clear message when binary not on PATH
+
+**Given:**
+- `docs/agents/agent-cli.md` contains the missing binary fixture
+- `command -v totally-not-on-path` fails
+
+**When:** `implement-issue` validates the binary at invocation time
+
+**Then:** it fails immediately with:
+
+```
+Agent 'totally-not-on-path' not found on PATH — rerun /setup-skills to reconfigure.
+```
+
+**Notes:**
+- Validation happens before any tmux launch
+- Error message directs user to re-run setup-skills
+
+---
+
+### Test: reads selected agent from docs/agents/agent-cli.md
+
+**Given:**
+- `docs/agents/agent-cli.md` exists and contains valid YAML with `selected`, `binary`, and `args` fields
+
+**When:** `implement-issue` reads the config at invocation time
+
+**Then:** it parses the three fields correctly and uses them for dispatch
+
+**Notes:**
+- Does not use hardcoded dispatch table
+- Does not run detection scripts
+- Reads directly from the repo-side config file
+
+---
+
+### Test: @{prompt} placeholder substitution
+
+**Given:**
+- `docs/agents/agent-cli.md` contains opencode fixture (args: "run -f @{prompt}")
+- `command -v mise` succeeds
+
+**When:** `implement-issue` composes the tmux launch command with prompt file at `/tmp/issue-99-prompt.md`
+
+**Then:** the generated command contains:
+
+```
+opencode run -f /tmp/issue-99-prompt.md
+```
+
+**Notes:**
+- `@{prompt}` in args is replaced with the absolute path to the temp prompt file
+- Works regardless of where `@{prompt}` appears in the args string