chore: archive completed milestone phase directories

[RichardHightower]

Summary

Archives 47 completed phase directories from .planning/phases/ into .planning/milestones/v{X.Y}-phases/ archive directories. Pure housekeeping — no code changes.

Distribution:

• v3.0 (4): phases 01-04
• v6.0/v6.0.4 (7, shared archive): phases 05-11
• v7.0 (3): phases 12-14
• v8.0 (10): phases 15-17, 19-25
• v9.3.0 (2): phases 34-35
• v9.4.0 (10): phases 29-33, 36-40
• v9.5.0 (5): phases 41-45
• v9.6.0 (parked) (1): phase 46 (only one created; 47-49 deferred post-MCP)
• v10.2 (6): phases 50-55

After this PR, .planning/phases/ is empty — ready for v10.3 phase scoping.

Test plan

• task before-push passes (468 MCP tests, 92% coverage)
• git status clean on branch
• All git mv operations preserve file history
• No code changed — pure file relocation