Add milestone planning phase between /generate-app and /add-feature

- New command /plan-milestones: generates docs/milestones/ (roadmap.md + one document per milestone with its features, acceptance criteria, and ready-to-run /add-feature commands)
- New skill milestone-planning: MVP-first vertical-slice planning rules + milestone/roadmap templates
- /add-feature: reads the feature's milestone document before planning and checks the feature off (updating milestone/roadmap status) in Step 8
- Workflow diagrams, AGENTS.md, and README updated for the new phase

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.opencode/command/add-feature.md

@@ -27,6 +27,7 @@ agent: build
 
 1. Read `AGENTS.md` to grasp the overall picture of the project.
 2. Review the persistent documents in the `docs/` directory to understand the relevant design philosophy and architecture.
+3. If `docs/milestones/` exists, read `docs/milestones/roadmap.md` and find the milestone document that contains this feature. Use its description, related requirements, affected screens, and acceptance criteria as the basis for planning. If the feature is not in any milestone, note that it is unplanned work and proceed.
 
 ## Step 3: Investigate Existing Patterns
 
@@ -147,9 +148,14 @@ If any of the following situations occur while the implementation loop is runnin
    - ✅ `docs/development-guidelines.md` — add/update any new conventions, patterns, or standards established during the feature
    - ✅ `docs/glossary.md` — add/update any new domain terms introduced by the feature
 
-3. **Consistency check**: re-read each updated document and confirm it is consistent with the other documents and with the implemented code. Fix any inconsistencies found.
+3. **Update the milestone documents (if `docs/milestones/` exists)**:
+   - In the feature's milestone document, use the edit tool to check the feature off (`[ ]` → `[x]`).
+   - If this was the last unchecked feature of the milestone, set the milestone's status to `Completed` (in both the milestone document and `docs/milestones/roadmap.md`), fill in its "Completion Notes", and update the roadmap's current-milestone pointer to the next milestone.
+   - If the feature was not part of any milestone, add it to the current milestone's document as a completed (`[x]`) feature so the plan reflects reality.
 
-4. **Once this step completes successfully, never stop; immediately proceed to the Completion Criteria.**
+4. **Consistency check**: re-read each updated document and confirm it is consistent with the other documents and with the implemented code. Fix any inconsistencies found.
+
+5. **Once this step completes successfully, never stop; immediately proceed to the Completion Criteria.**
 
 ## Completion Criteria
 
@@ -157,6 +163,6 @@ This workflow completes automatically once all of the following conditions are m
 - Step 5: All tasks in `tasklist.md` are complete (`[x]` or skipped for a valid reason).
 - Step 6: The `implementation-validator` subagent's validation passes.
 - Step 7: The `test`, `lint`, and `typecheck` commands all succeed without errors.
-- Step 8: Handover notes are recorded in `tasklist.md`, and all six persistent documents in `docs/` have been reconciled with the feature (updates applied, or a skip reason noted for each).
+- Step 8: Handover notes are recorded in `tasklist.md`, all six persistent documents in `docs/` have been reconciled with the feature (updates applied, or a skip reason noted for each), and the feature is checked off in its milestone document (if `docs/milestones/` exists).
 
 Until these completion criteria are met, continue to think autonomously, solve problems, and carry on the work.