Restructure milestone plan into phase files; /execute-milestones now requires a phase file

- /plan-milestones groups milestones into phases and generates one file per phase (phase1-milestones.md, phase2-milestones.md, ...) plus roadmap.md; milestone numbering stays global across phases
- /execute-milestones takes a required phase file argument (phase1-milestones.md | full path | phase1 | 1) and executes only that file's milestones one by one; with no argument it lists available phase files and stops; closes the phase in the roadmap when done
- milestone-planning skill/template rewritten around the Phase > Milestone > Feature hierarchy; milestone-execution skill scoped to one phase file per run
- /add-feature, AGENTS.md, README, and docs/milestones/README.md updated to phase-file references

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

AGENTS.md

@@ -87,9 +87,9 @@ Define "what to build" and "how to build it" for the entire application:
 - Created by `/define-design`, updated by `/update-design`, consumed by `/generate-app` and `/add-feature`
 
 #### Milestone Documents (`docs/milestones/`)
-- **roadmap.md** - Milestone overview: table, ordering rationale, current milestone, icebox
-- **milestone-[NN]-[name].md** - One per milestone: goal, scope, and the features it contains (each sized for one `/add-feature` run)
-- Created by `/plan-milestones`; `/add-feature` checks features off as it completes them
+- **roadmap.md** - Overview: phase/milestone table, ordering rationale, current phase and milestone, icebox
+- **phase[N]-milestones.md** - One per phase (the `/execute-milestones` unit): its milestones and their features (each feature sized for one `/add-feature` run)
+- Created by `/plan-milestones`; `/execute-milestones` and `/add-feature` check features off as they complete them
 
 ### Work-Unit Documents (`.steering/`)
 
@@ -114,9 +114,9 @@ Define "what to do this time" for a specific development task:
 3. Define the UI/UX design with `/define-design` (creates `docs/design/`)
 4. Review and refine the UI/UX with `/update-design` (or re-run `/define-design`) until approved — before generating code
 5. Generate the initial app with `/generate-app web | flutter | winui3`
-6. Plan milestones with `/plan-milestones` (creates `docs/milestones/` with the features for each milestone)
+6. Plan milestones with `/plan-milestones` (creates `docs/milestones/` with one `phase[N]-milestones.md` per phase)
 7. Implement, either way:
-   - `/execute-milestones` — execute all milestones one by one autonomously (one steering directory per milestone), or
+   - `/execute-milestones phase1-milestones.md` — execute one phase file: its milestones one by one autonomously (one steering directory per milestone); repeat with the next phase file, or
    - `/add-feature [feature]` — one feature at a time, milestone by milestone
 
 ### Day-to-Day Usage
@@ -139,9 +139,9 @@ Define "what to do this time" for a specific development task:
 
 # Milestone planning and execution
 > /plan-milestones
-> /plan-milestones 3 milestones, MVP first
-> /execute-milestones
-> /execute-milestones 1
+> /plan-milestones 2 phases, MVP first
+> /execute-milestones phase1-milestones.md
+> /execute-milestones phase2-milestones.md
 
 # Detailed review (when a detailed report is needed)
 > /review-docs docs/product-requirements.md