refactor(rules): establish canonical PR review-cycle and severity sources

Establishes two canonical rule sources to eliminate drift between local
taskmaestro skill, shared rules, adapter docs, and custom instructions:

- rules/severity-classification.md — canonical severity taxonomy with two
  distinct scales: Code Review Severity (critical/high/medium/low) for PR
  approval gates, and Production Incident Severity (P1-P4) for on-call /
  SLO burn rate. Includes explicit mapping guidance so the two are not
  conflated.
- rules/pr-review-cycle.md — canonical PR review cycle protocol moved from
  .claude/skills/taskmaestro/SKILL.md L86-430. Covers CI gate, routing
  (Conductor Review default / Review Agent via --review-pane), the review
  protocol, worker response, re-review, approval criteria, max 3 cycles,
  and commit hygiene (amend + force-with-lease).

Aligns surrounding files to reference the canonical sources instead of
duplicating protocol text:

- .claude/skills/taskmaestro/SKILL.md — L86-430 replaced with a concise
  summary + canonical link. Retains taskmaestro-specific implementation
  details (file-based tmux trigger, taskmaestro-state.json schema).
- packages/rules/.ai-rules/skills/incident-response/severity-classification.md
  — P1-P4 definitions removed (now in canonical). Keeps operational
  content: burn rate math, SLO tier mapping, decision tree, report template.
- packages/rules/.ai-rules/skills/pr-review/SKILL.md — adds note linking
  Priority Levels to canonical Code Review Severity.
- packages/rules/.ai-rules/adapters/claude-code.md — adds cross-reference
  to canonical sources near Exit Criteria. Does NOT touch Nested Execution
  Examples area (reserved for Wave 4 #1386).

Option D Hybrid chosen because it preserves the taskmaestro skill's
operational value while giving the repo one authoritative place for
review-cycle and severity semantics.

Verification:
- markdownlint-cli2 0 errors across 103 files
- yarn workspace codingbuddy lint (0 errors, 1 pre-existing warning)
- yarn workspace codingbuddy typecheck
- yarn workspace codingbuddy test:coverage (237 files, 5947 tests passed)
- yarn workspace codingbuddy circular (0 circular deps)
- yarn workspace codingbuddy build
- ajv-cli validate all agent JSON schemas

Closes #1385

Author: JeremyDev87