refactor: lean out SKILL.md — core under 250 lines, 5 reference docs

[rmichaelthomas]

Splits the 699-line SKILL.md into a ~200-line core (loaded every session) plus 5 reference documents in references/ (read on demand). No behavioral changes — structural refactor only. The core covers: two-channel protocol, list declaration rule, cite-before-claiming, abbreviated session end, tiers, vocabulary constraint, session pack summary. References cover: save procedure (classifier handling, payload fields, tier fallbacks), session corrections (full table + consultation checklist), pre-commit gate, starting a contract (inheritance, template, source documents), and contradiction check. Part of P2-10 enterprise readiness (per-seat token cost reduction).

Spec divergences (resolved by judgment, no fabrication)

The build prompt was written against a slightly different SKILL.md version. Resolved against the actual file (697 lines) to honor the no-content-loss invariant:

• No sensitivity-check step / docs/TRUST-BOUNDARY.md / docs/LOCAL-ONLY.md exist in the current file. The spec described them; I did not fabricate them. Session end stays a faithful 3-step sequence (emit → save → close).
• "Install — hook & statusline" and session-pack usage example had no named target doc. Install + persistence folded into starting-a-contract.md; the session-pack section stayed in core (no target doc, and it is short).
• parent_id discovery is canonical in save-procedure.md; starting-a-contract.md links to it (no duplication).

Verification

• Core SKILL.md: 217 lines (target <250)
• Concatenation of all 6 output files: 745 lines (≥699, no content lost)
• All references/*.md links resolve
• Signature phrases from each moved section confirmed present in exactly one file

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