feat(executor): add cross-AI execution hook (step 2.5) in execute-phase (#1875)

Add optional cross-AI delegation step that lets execute-phase delegate
plans to external AI runtimes via stdin-based prompt delivery. Activated
by --cross-ai flag, plan frontmatter cross_ai: true, or config key
workflow.cross_ai_execution. Adds 3 config keys, template defaults,
and 18 tests.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

get-shit-done/workflows/execute-phase.md

@@ -57,6 +57,8 @@ Parse `$ARGUMENTS` before loading any context:
 - First positional token → `PHASE_ARG`
 - Optional `--wave N` → `WAVE_FILTER`
 - Optional `--gaps-only` keeps its current meaning
+- Optional `--cross-ai` → `CROSS_AI_FORCE=true` (force all plans through cross-AI execution)
+- Optional `--no-cross-ai` → `CROSS_AI_DISABLED=true` (disable cross-AI for this run, overrides config and frontmatter)
 
 If `--wave` is absent, preserve the current behavior of executing all incomplete waves in the phase.
 </step>
@@ -243,6 +245,77 @@ Report:
 ```
 </step>
 
+<step name="cross_ai_delegation">
+**Optional step 2.5 — Delegate plans to an external AI runtime.**
+
+This step runs after plan discovery and before normal wave execution. It identifies plans
+that should be delegated to an external AI command and executes them via stdin-based prompt
+delivery. Plans handled here are removed from the execute_waves plan list so the normal
+executor skips them.
+
+**Activation logic:**
+
+1. If `CROSS_AI_DISABLED` is true (`--no-cross-ai` flag): skip this step entirely.
+2. If `CROSS_AI_FORCE` is true (`--cross-ai` flag): mark ALL incomplete plans for cross-AI execution.
+3. Otherwise: check each plan's frontmatter for `cross_ai: true` AND verify config
+   `workflow.cross_ai_execution` is `true`. Plans matching both conditions are marked for cross-AI.
+
+```bash
+CROSS_AI_ENABLED=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow.cross_ai_execution --default false 2>/dev/null)
+CROSS_AI_CMD=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow.cross_ai_command --default "" 2>/dev/null)
+CROSS_AI_TIMEOUT=$(node "$HOME/.claude/get-shit-done/bin/gsd-tools.cjs" config-get workflow.cross_ai_timeout --default 300 2>/dev/null)
+```
+
+**If no plans are marked for cross-AI:** Skip to execute_waves.
+
+**If plans are marked but `cross_ai_command` is empty:** Error — tell user to set
+`workflow.cross_ai_command` via `gsd-tools.cjs config-set workflow.cross_ai_command "<command>"`.
+
+**For each cross-AI plan (sequentially):**
+
+1. **Construct the task prompt** from the plan file:
+   - Extract `<objective>` and `<tasks>` sections from the PLAN.md
+   - Append PROJECT.md context (project name, description, tech stack)
+   - Format as a self-contained execution prompt
+
+2. **Check for dirty working tree before execution:**
+   ```bash
+   if ! git diff --quiet HEAD 2>/dev/null; then
+     echo "WARNING: dirty working tree detected — the external AI command may produce uncommitted changes that conflict with existing modifications"
+   fi
+   ```
+
+3. **Run the external command** from the project root, writing the prompt to stdin.
+   Never shell-interpolate the prompt — always pipe via stdin to prevent injection:
+   ```bash
+   echo "$TASK_PROMPT" | timeout "${CROSS_AI_TIMEOUT}s" ${CROSS_AI_CMD} > "$CANDIDATE_SUMMARY" 2>"$ERROR_LOG"
+   EXIT_CODE=$?
+   ```
+
+4. **Evaluate the result:**
+
+   **Success (exit 0 + valid summary):**
+   - Read `$CANDIDATE_SUMMARY` and validate it contains meaningful content
+     (not empty, has at least a heading and description — a valid SUMMARY.md structure)
+   - Write it as the plan's SUMMARY.md file
+   - Update STATE.md plan status to complete
+   - Update ROADMAP.md progress
+   - Mark plan as handled — skip it in execute_waves
+
+   **Failure (non-zero exit or invalid summary):**
+   - Display the error output and exit code
+   - Warn: "The external command may have left uncommitted changes or partial edits
+     in the working tree. Review `git status` and `git diff` before proceeding."
+   - Offer three choices:
+     - **retry** — run the same plan through cross-AI again
+     - **skip** — fall back to normal executor for this plan (re-add to execute_waves list)
+     - **abort** — stop execution entirely, preserve state for resume
+
+5. **After all cross-AI plans processed:** Remove successfully handled plans from the
+   incomplete plan list so execute_waves skips them. Any skipped-to-fallback plans remain
+   in the list for normal executor processing.
+</step>
+
 <step name="execute_waves">
 Execute each selected wave in sequence. Within a wave: parallel if `PARALLELIZATION=true`, sequential if `false`.