mgw:project should support extending completed projects

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Problem

When all milestones in a project are complete, running /mgw:project exits with "Project already initialized." The only documented way to add new milestones is to delete .mgw/ entirely and re-initialize, losing all local tracking history (cross-refs, completed archives, config).

Expected Behavior

/mgw:project should detect when all milestones are complete and offer to extend the project with new milestones instead of blocking:

• Preserve existing project.json data (completed milestones, cross-refs, config)
• Append new milestones to the milestones[] array
• Set current_milestone to the first new milestone
• Reuse the existing GitHub Projects v2 board instead of creating a new one
• Continue phase numbering from where the last milestone left off (e.g., if v5.0 ended at Phase 42, new milestones start at Phase 43)

Scope

• Modify project.md step verify_repo to detect "all milestones done" state
• Add extend flow: gather inputs → generate → append to project.json
• Update create_project_board to reuse existing board when project.project_board exists
• Update write_project_json to merge instead of overwrite

Related

• Phase numbering continuation (#TBD)
• Project history awareness for template generation (#TBD)

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MGW · triage-complete · 2026-02-27T07:27:45Z

Triage Complete

Scope	large — 7 files across project initialization, state management, template validation, template schema, milestone execution, next issue resolution, user documentation
Validity	confirmed
Security	none
Route	gsd:quick --full — 7 files extending established patterns; full plan+execute+verify without overkill of full milestone
Conflicts	#56 (dependent), #57 (minor overlap), #58 (related), #59 (tangential)

Work begins on branch issue/55-mgw-project-extend.

Affected Files

• commands/project.md — add extend flow branch in verify_repo
• lib/state.cjs — merge vs overwrite for project state
• lib/template-loader.cjs — validate against existing milestones
• templates/schema.json — no structural changes needed
• commands/milestone.md — verify indexing with appended milestones
• commands/next.md — verify boundary detection after extend
• docs/USER-GUIDE.md — document extend behavior

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MGW · execution-complete · 2026-02-27T07:44:20Z

Execution Complete

4 atomic commit(s) on branch issue/55-mgw-project-extend.

Changes: 4 files changed, 317 insertions(+), 21 deletions(-)

Preparing pull request.

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MGW · pr-ready · 2026-02-27T07:47:00Z

PR Ready

PR #67 — #67

Add project extension support — when all milestones are complete, /mgw:project detects this and offers to extend with new milestones, preserving existing state, continuing phase numbering, and reusing the GitHub Projects board.

Testing procedures posted on the PR.
This issue will auto-close when the PR is merged.

Pipeline Summary

Stage	Status
Triage	✓
Planning	✓
Plan Check	✓ (all checks passed)
Execution	✓ (3 tasks, 4 commits)
Verification	✓ (7/7 must-haves)
PR Creation	✓