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, CLAUDE.md, README, and settings.json updated for the new phase

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

docs/milestones/README.md

@@ -0,0 +1,40 @@
+# Milestone Documents (`docs/milestones/`)
+
+This directory holds the project's **milestone plan**: an ordered breakdown of the product into releases, with the features for each milestone. It is created by `/plan-milestones` and kept up to date by `/add-feature`.
+
+## Structure
+
+```
+docs/milestones/
+├── roadmap.md                      # Overview: milestone table, ordering rationale, icebox
+├── milestone-01-[name].md          # Milestone 1 (the MVP)
+├── milestone-02-[name].md
+└── milestone-NN-[name].md
+```
+
+## File purposes
+
+### `roadmap.md`
+The overview: a table of all milestones (goal, feature count, status), the ordering rationale, a pointer to the current milestone, and a "Later / Icebox" list of consciously deferred items.
+
+### `milestone-[NN]-[name].md`
+One document per milestone. Contains the milestone's goal, scope, dependencies, and its **feature list** — each feature sized for exactly one `/add-feature` run, with description, related PRD requirements, affected screens, acceptance criteria, and the ready-to-run command.
+
+## Status tracking
+
+- Each feature has a checkbox in its milestone document; `/add-feature` checks it off (`[ ]` → `[x]`) when the feature completes.
+- Milestone status (`Not started` / `In progress` / `Completed`) is kept in sync between the milestone document and the roadmap table.
+
+## Workflow
+
+```
+/setup-project     → product/architecture docs
+/define-design     → UI/UX design spec
+/generate-app      → initial app shell
+/plan-milestones   → creates the files above
+/add-feature       → implements one feature, then checks it off here
+```
+
+## Status
+
+This directory is populated by running `/plan-milestones`. Until then, the files above do not exist.