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/skills/milestone-planning/SKILL.md

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+---
+name: milestone-planning
+description: Guide for planning development milestones and generating milestone documents with a feature breakdown. Use when planning milestones, a release roadmap, or deciding implementation order under docs/milestones/.
+---
+
+# Milestone Planning Skill
+
+This skill explains how to break a product into ordered milestones and produce milestone documents under `docs/milestones/`. The documents are the bridge between the specification (`docs/`) and implementation (`/add-feature`): every feature in every milestone is sized to be implemented with **one `/add-feature` run**.
+
+## Core Principles
+
+1. **MVP first**: Milestone 1 is the smallest coherent, end-to-end usable product. Not a layer (e.g. "all the models"), but a vertical slice a user could actually use.
+2. **Vertical slices**: Each feature cuts through UI → logic → data. Avoid horizontal milestones like "backend milestone" / "frontend milestone".
+3. **One feature = one `/add-feature` run**: If a feature needs more than one steering directory to implement, split it. If it is a one-line change, merge it into a neighbor.
+4. **Dependency order**: A feature never lands in an earlier milestone than something it depends on.
+5. **Exactly-once assignment**: Every feature belongs to exactly one milestone. Deliberately deferred items go to the roadmap's "Later / Icebox" — nothing silently disappears.
+6. **Living documents**: Milestone documents track status. `/add-feature` checks off a feature (`[ ]` → `[x]`) when it completes, and the roadmap status is updated when a milestone finishes.
+
+## Inputs
+
+Before planning, read:
+
+- `docs/product-requirements.md` — user stories, priorities (P0/P1/P2), success metrics
+- `docs/functional-design.md` — functional modules and data flows
+- `docs/architecture.md` — technical constraints and dependencies
+- `docs/design/ui-blueprint.json` (if present) — screens and user actions
+- `docs/ideas/*` (if present)
+
+## Outputs
+
+```
+docs/milestones/
+├── roadmap.md                      # Overview: milestone table, ordering rationale, icebox
+├── milestone-01-[name].md          # MVP
+├── milestone-02-[name].md
+└── milestone-NN-[name].md
+```
+
+File naming: `milestone-[NN]-[kebab-case-name].md` (e.g. `milestone-01-mvp.md`, `milestone-02-collaboration.md`).
+
+Use the structure in `template.md` for each milestone document.
+
+## How to Slice Features
+
+A well-sized feature:
+
+- Delivers observable user value (or unblocks one that does)
+- Touches one primary screen/flow (may touch shared components)
+- Has 3–7 acceptance criteria
+- Can be described in one sentence starting with a verb ("Edit user profile", "Filter tasks by tag")
+- Works verbatim as a command argument: `/add-feature Edit user profile`
+
+Too big (split it): "User management" → "Register account", "Log in / log out", "Edit user profile", "Reset password".
+Too small (merge it): "Change button color" → fold into the feature that owns that screen.
+
+## How to Group Milestones
+
+1. Tag every feature with its PRD priority and its dependencies.
+2. Milestone 1 (MVP): the minimal dependency-closed set of P0 features that forms a usable product walk-through (a user can start, do the core job, and finish).
+3. Following milestones: group remaining features by theme and priority, keeping each milestone a meaningful release on its own ("after milestone N ships, the user can additionally …").
+4. Prefer 2–4 milestones with 3–8 features each. More than ~8 features in one milestone usually means it should be split.
+
+## Status Tracking Rules
+
+- Feature status lives **only** in the feature checkbox of its milestone document.
+- Milestone status (`Not started` / `In progress` / `Completed`) lives in both the milestone document header and the roadmap table — keep them in sync.
+- `/add-feature` Step 8 updates the feature checkbox and, when the last feature of a milestone completes, the milestone and roadmap statuses.
+- Re-planning (adding/moving/removing features) must update both the milestone document(s) and `roadmap.md` in the same change.
+
+## Consistency Checklist
+
+Before finishing:
+
+- [ ] Every P0/P1 PRD requirement maps to a feature in some milestone (or is explicitly in the icebox).
+- [ ] Every feature appears in exactly one milestone.
+- [ ] No feature precedes one of its dependencies.
+- [ ] Milestone 1 is a usable end-to-end product on its own.
+- [ ] Every feature has acceptance criteria and a ready-to-run `/add-feature` command line.
+- [ ] `roadmap.md` table matches the milestone documents (names, counts, statuses).