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

docs/agents/agent-cli.md

@@ -0,0 +1,23 @@
+# Agent CLI Configuration
+
+The CLI agent used by `/implement-issue` for spawning child agents in isolated git worktrees.
+
+## Configuration
+
+```yaml
+selected: pi
+binary: pi
+args: "-p @{prompt}"
+```
+
+## Fields
+
+| Field | Description |
+|-------|-------------|
+| `selected` | Display name of the selected agent |
+| `binary` | Command name expected on PATH |
+| `args` | Static arguments. `@{prompt}` is substituted with the prompt file path at invocation time. Empty means stdin piping via `cat`. |
+
+## Changing the agent
+
+Run `/setup-skills` Section E to reconfigure. The full list of supported agents is in `common/engineering/setup-skills/agents-seed.md`.