fix: harden forge-coordination skill against DCP compression

[jflowers]

Summary

Restructures the forge-coordination skill to survive DCP context compression. Critical access-control constraints were expressed as parenthetical asides and buried bullet items — exactly the constructs that context compression drops first.

Addresses four specific fragilities identified in #48:

1. (coordinator only) parenthetical on comms_release_all() — promoted to ## Coordinator-Only Operations section header
2. exclusive=true as optional bullet — made the documented default in comms_reserve call signature
3. Conflict resolution in a separate section — inlined at the "Reserve files" step in the Worker Protocol
4. Mixed coordinator/worker rules in a flat list — split into role-scoped sections with MUST/MUST NOT rules at section top

Closes #48

How to Test

# Build compiles with updated embedded skill
make build

# All tests pass (190+ tests, race detection)
make test

# Both skill copies are byte-identical
diff internal/agentkit/content/skills/forge-coordination/SKILL.md \
     .opencode/skills/forge-coordination/SKILL.md

# Verify the restructured document
cat internal/agentkit/content/skills/forge-coordination/SKILL.md

Verify the following structural properties in the output:

• ## Coordinator-Only Operations section exists with MUST NOT rules before protocol steps
• ## Worker-Only Operations section exists with MUST/MUST NOT rules before protocol steps
• comms_release_all() appears only under Coordinator-Only Operations
• Workers MUST NOT call comms_release_all() is explicit in Worker-Only Operations
• Conflict resolution steps are inlined after step 3 (Reserve files) in Worker Protocol
• comms_reserve(paths=[...], exclusive=true, reason="...") is the documented call

How to Demo

Review the restructured skill document side-by-side with the original (via git diff). The key observation is that safety constraints have moved from low-salience positions (parentheticals, buried bullets, separate sections) to high-salience positions (section headers, first-position MUST rules, inline at point of use).

Key Files Changed

Skill files (implementation):

• .opencode/skills/forge-coordination/SKILL.md — restructured with role-scoped sections
• internal/agentkit/content/skills/forge-coordination/SKILL.md — identical copy (embedded in binary)

OpenSpec artifacts (planning):

• openspec/changes/harden-forge-skill-compression/proposal.md — motivation, impact, constitution alignment
• openspec/changes/harden-forge-skill-compression/design.md — 5 design decisions with rationale
• openspec/changes/harden-forge-skill-compression/specs/forge-coordination-hardening.md — delta spec with GIVEN/WHEN/THEN scenarios
• openspec/changes/harden-forge-skill-compression/tasks.md — implementation checklist (all complete)

Known Issues

The following findings from the review council were acknowledged but not resolved (out of scope for this change):

• LOW: forge-global skill uses weaker phrasing for comms_release_all access control
• LOW: /handoff command calls comms_release_all() without role guard
• LOW: Worker agent checklist omits explicit comms_release step

This PR was generated by /uf.finale (AI-assisted).