Add /execute-milestones to run milestones one by one with per-milestone steering docs

- New command /execute-milestones: executes all (or selected) milestones strictly in order; for each milestone creates a separate .steering/[date]-milestone-NN-[name]/ directory, divides its features into tasks grouped by feature, implements, validates, tests, reconciles docs, and updates milestone/roadmap statuses before starting the next
- New skill milestone-execution: per-milestone steering structure, task-division rules, gate criteria between milestones, and failure handling
- plan-milestones completion message, AGENTS.md, README, and docs/milestones/README.md updated

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

README.md

@@ -49,8 +49,19 @@ a `roadmap.md` overview plus one document per milestone listing **the features i
 each sized for exactly one `/add-feature` run, with acceptance criteria and a ready-to-run command.
 Milestone 1 is the MVP; later milestones build on it by priority and dependency.
 
-### Step 7 — Implement features: `/add-feature <feature>`
-A fully autonomous loop, one feature at a time, milestone by milestone. Each run:
+### Step 7 — Implement the milestones
+
+Two ways to work through the plan:
+
+**Option A — execute whole milestones: `/execute-milestones`**
+Runs all remaining milestones **one by one, autonomously**. For each milestone it creates a
+**separate steering directory** (`.steering/[date]-milestone-[NN]-[name]/`), divides the
+milestone's features into concrete tasks grouped by feature, implements them all, validates,
+tests, reconciles the docs, and marks the milestone completed — then immediately starts the next
+milestone. Takes an optional target (`/execute-milestones 1` or `/execute-milestones 2-3`).
+
+**Option B — one feature at a time: `/add-feature <feature>`**
+A fully autonomous loop, one feature per run. Each run:
 
 1. Creates `.steering/[YYYYMMDD]-[feature]/` (requirements, design, tasklist)
 2. Reads the feature's milestone document and the design spec first — and updates the design first if the feature changes the UI
@@ -60,7 +71,7 @@ A fully autonomous loop, one feature at a time, milestone by milestone. Each run
 6. Records a retrospective, reconciles all six `docs/` documents, and checks the feature off in its milestone document
 
 ### Step 8 — Repeat and maintain
-- More features → `/add-feature <feature>`, working through the milestones in order
+- Remaining milestones → `/execute-milestones`, or feature by feature with `/add-feature <feature>`
 - UI changes → `/update-design <change>` first, then `/add-feature`
 - Re-planning → re-run `/plan-milestones` or edit `docs/milestones/` in conversation
 - Document quality checks → `/review-docs <path>`
@@ -212,6 +223,7 @@ opencode
     │   ├── generate-app.md
     │   ├── update-design.md
     │   ├── plan-milestones.md
+    │   ├── execute-milestones.md
     │   ├── add-feature.md
     │   └── review-docs.md
     └── skills/                # Task-specific skills
@@ -225,6 +237,7 @@ opencode
         ├── design-tokens/         (SKILL.md)
         ├── platform-ui-generation/(SKILL.md)
         ├── milestone-planning/    (SKILL.md + template.md)
+        ├── milestone-execution/   (SKILL.md)
         └── steering/              (SKILL.md + templates/)
 ```
 
@@ -359,7 +372,21 @@ This fully autonomous workflow:
 7. Runs `npm test`, `npm run lint`, `npm run typecheck`
 8. Records a retrospective, reconciles all six persistent docs with the feature, and checks the feature off in its milestone document
 
-### 6. Review a document
+### 6. Execute all milestones one by one (alternative to per-feature runs)
+
+```
+> /execute-milestones            # all remaining milestones, in order
+> /execute-milestones 1          # only milestone 1
+> /execute-milestones 2-3       # milestones 2 through 3
+```
+
+Works through the milestone plan autonomously, one milestone at a time. For each milestone it
+creates a **separate steering directory** (`.steering/[date]-milestone-[NN]-[name]/`), divides the
+milestone's features into tasks grouped by feature, implements everything, validates, runs the
+test suite, reconciles the docs, marks the milestone `Completed` — and immediately starts the next
+one. It only stops early on a genuine external blocker, which it records in the roadmap.
+
+### 7. Review a document
 
 ```
 > /review-docs docs/product-requirements.md
@@ -368,7 +395,7 @@ This fully autonomous workflow:
 Launches the `doc-reviewer` subagent to evaluate the document from five perspectives:
 completeness, clarity, consistency, implementability, and measurability.
 
-### 7. Day-to-day editing
+### 8. Day-to-day editing
 
 You don't always need a command — just ask in normal conversation:
 
@@ -393,7 +420,7 @@ Recommended workflow:
    - Run `/update-design <change>` for targeted refinements
 4. Run `/generate-app web`, `/generate-app flutter`, or `/generate-app winui3`
 5. Run `/plan-milestones` to break the product into milestones with a feature list each
-6. Use `/add-feature` to add features, milestone by milestone
+6. Use `/execute-milestones` to implement all milestones one by one, or `/add-feature` to go feature by feature
 7. Use `/update-design` whenever a feature changes the UI
 
 Source of truth:
@@ -416,6 +443,7 @@ See [`docs/design/README.md`](docs/design/README.md) for the full structure.
 | `/generate-app` | Scaffold the initial app for a target platform | `/generate-app web` |
 | `/update-design` | Update the design spec when a feature changes the UI | `/update-design add a profile screen` |
 | `/plan-milestones` | Plan milestones and generate milestone documents with the features for each | `/plan-milestones 3 milestones, MVP first` |
+| `/execute-milestones` | Execute milestones one by one, with separate steering documents per milestone | `/execute-milestones` |
 | `/add-feature` | Implement a feature end-to-end (autonomous) | `/add-feature User profile editing` |
 | `/review-docs` | Detailed document review via subagent | `/review-docs docs/architecture.md` |
 
@@ -435,6 +463,7 @@ See [`docs/design/README.md`](docs/design/README.md) for the full structure.
 | `design-tokens` | Creating and maintaining design tokens | `docs/design/design-tokens.json` |
 | `platform-ui-generation` | Converting design files into Web/Flutter/WinUI 3 code | generated app code |
 | `milestone-planning` | Planning milestones and the feature breakdown per milestone | `docs/milestones/` files |
+| `milestone-execution` | Executing milestones one by one with per-milestone steering docs | `.steering/[date]-milestone-NN-[name]/` files |
 | `steering` | Planning, implementing, and retrospecting on a task | `.steering/[date]-[name]/` files |
 
 Each skill folder contains a `SKILL.md` (instructions) plus `template.md` and/or `guide.md` (reference