docs(maestro-case): split validate + ship into dedicated Phase 4 + 5

[song-zhao-25]

Summary

• Agents occasionally skipped debug + validate after Phase 2b's "Full validate" step because the post-build prompt was framed as optional ("Optional: Debug session") and buried at the tail of the detail-build phase. This refactor promotes validate and ship to their own dedicated phases so they cannot be silently skipped.
• Rename: Phase 2a → Phase 2 (skeleton), Phase 2b → Phase 3 (detail). All 17 plugin/reference docs updated.
• New Phase 4 — Validate (mandatory, runs after Phase 3). Agent fixes failures via direct caseplan.json edits (no Phase 3 plugin re-runs — agent has enough context to make targeted fixes). Cap at 3 retries; on 3rd failure, AskUserQuestion Retry with fix / Pause for manual edit / Abort.
• New Phase 5 — Ship (mandatory hard-stop loop). Same prompt-discipline as the Phase 2 → 3 stop. Debug + publish are options inside the loop, not separate phases. Debug failure recovery mirrors Phase 4: agent triages from case debug error message, edits caseplan.json, re-runs debug. Cap 3 retries.
• Rule 14 added to SKILL.md to enforce Phase 4 + 5 always-runs contract — closes the agent-skip gap.
• Phases 1–3 are forward-only (best-effort, graceful degradation, no back-edges). Only Phase 4 and Phase 5 have failure-recovery loops, and those recover via direct edits — not by re-running earlier phases.

Test plan

• Walk a sample case through the skill end-to-end; confirm Phase 4 validate runs unprompted and Phase 5 ship prompt fires every time
• Trigger a Phase 4 validation failure; confirm agent edits caseplan.json directly (no Phase 3 plugin re-invocation) and re-validates
• Confirm Phase 5 prompt loops correctly through debug → publish → done and publish → debug → done
• Trigger a debug runtime error; confirm agent triages and re-runs debug up to 3 retries
• Confirm Phase 2a / Phase 2b no longer appear anywhere in skill docs (including plugin impl-json.md files)
• Confirm Rule 14 blocks any code path that would skip Phase 4 or Phase 5

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