feat: MEGG v1.2.0 — stale warning, init --update, universal entry types

[ToruGuy]

Summary

• Stale info.md warning — context() warns when info.md hasn't been updated in >30 days, prompts init --update
• init --update mode — intelligent section-by-section review of existing info.md instead of returning "already initialized"
• Universal entry types — new taxonomy that works for any domain, not just code:
  • rule — always/never do X (replaces pattern + gotcha)
  • fact — this is true about X (replaces context)
  • decision — we chose X because Y (unchanged)
  • process — how to do X step by step (new)

Breaking Changes

Entry types pattern, gotcha, context removed. Existing knowledge.md files with old types still parse correctly — only new entries must use new types.

Test plan

• npm run build — clean compile
• npm test — 42/42 tests pass
• context() shows stale warning when info.md > 30 days old
• init() on existing .megg triggers update flow (not "already initialized")
• learn() accepts rule, fact, decision, process — rejects old types