[P3-T02] forge migrate command for v0.2.x → v0.4 upgrade

[firatcand]

Forge task ID: P3-T02
Phase: 3 — Launch v0.4 · Type: backend · Owner: backend-dev · Priority: P0 · Estimate: M
Depends on: P2.5-T01 (#127), P2.5-T05 (#131)

src/cli/migrate.ts: detects v0.2.x drift signatures:

• @inherit lines in DESIGN.md
• /push-to-linear refs (renamed in v0.3)
• Missing .forge/settings.yaml
• References to dropped verbs in custom skills if any (forge orchestrate next, suggest-next, session-check, intent-detect)

Generates proposed diff with chalk preview. Updates SettingsSchema (adds codex/decisions/doctor blocks); rewrites skill call sites for renamed verbs (next → claim, suggest-next refs → phases --ready); archives any orphan workflow state files; adds templates/adr.template.md if missing. Backs up to .forge/backup-<timestamp>/. Applies on accept.

Acceptance:

• Detects all v0.2.x drift signatures on a fixture project
• Migration rewrites settings.yaml to include codex/decisions/doctor blocks
• Migration rewrites skill call sites for renamed verbs
• Backup directory contains pristine pre-migration originals
• Aborts cleanly on user reject; no partial state
• Re-runnable safely (idempotent)