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, CLAUDE.md, README, docs/milestones/README.md, and settings.json updated

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

README.md

@@ -48,8 +48,19 @@ 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
@@ -59,7 +70,7 @@ 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>`
@@ -99,7 +110,8 @@ claude
 > /update-design [refinement]      # repeat until the design is approved
 > /generate-app web                 # or flutter | winui3
 > /plan-milestones
-> /add-feature [feature name]       # milestone by milestone
+> /execute-milestones               # all milestones one by one, or:
+> /add-feature [feature name]       # one feature at a time
 ```
 
 See [Development Workflow (Step by Step)](#development-workflow-step-by-step) above for what each step does.
@@ -113,5 +125,6 @@ See [Development Workflow (Step by Step)](#development-workflow-step-by-step) ab
 | `/update-design` | Refine the design before generation, or update it when a feature changes the UI | `/update-design add a profile screen` |
 | `/generate-app` | Scaffold the initial app for a target platform | `/generate-app web` |
 | `/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` |