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/$SHELL -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/setup-skills/agents-seed.md

@@ -0,0 +1,45 @@
+# Agent CLI Seed Data
+
+This file is the source of truth for all supported CLI agents. It is read by `setup-skills` Section E to present the menu and by `implement-issue` for agent dispatch.
+
+## Agents
+
+```yaml
+agents:
+  - name: pi
+    binary: pi
+    args: "-p @{prompt}"
+    description: "My primary agent harness. Accepts prompt file via -p flag."
+
+  - name: opencode
+    binary: opencode
+    args: "run -f @{prompt}"
+    description: "OpenCode agent. Accepts prompt file via -f flag."
+
+  - name: goose
+    binary: goose
+    args: "run -i @{prompt}"
+    description: "Goose agent. Accepts prompt file via -i flag."
+
+  - name: codex
+    binary: codex
+    args: ""
+    description: "OpenAI Codex. Pipes stdin via cat."
+    note: "Uses stdin piping: args must be empty, prompt via pipe."
+
+  - name: claude
+    binary: claude
+    args: "-p"
+    description: "Anthropic Claude CLI. Pipes stdin via cat."
+    note: "Uses stdin piping: args is just the prompt flag, prompt via pipe."
+```
+
+## Field meanings
+
+| Field | Description |
+|-------|-------------|
+| `name` | Display name used in menus and `--agent` flag |
+| `binary` | Command name expected on PATH |
+| `args` | Static arguments appended after the binary. May include `@{prompt}` which is substituted at invocation time with the absolute path to the prompt file. Empty string means stdin piping (prompt is piped via `cat`). |
+| `description` | Short human-readable description for the setup menu |
+| `note` | Optional additional context |