fix: harden coordinator prompt against context compression

[jflowers]

Summary

Fixes #46 — the coordinator agent prompt (internal/agentkit/content/agents/coordinator.md) was compression-fragile. Its 6 behavioral constraints were expressed as unordered bullet points in a single "Rules" section, where critical constraints like "Never reserve files" and "Review every worker completion" would be lost under DCP context compression.

This restructures the prompt using patterns proven in the project's other agent files and the forge coordination skill:

• Identity-first opening embedding key constraints ("NEVER reserve files or edit code directly") in the first sentence
• Dedicated Critical Constraints section with uppercase MUST/NEVER keywords, positioned before the workflow
• Numbered 8-step Protocol replacing the flat bullet list, with explicit forge_review → forge_complete ordering
• Structural regression test (TestCoordinatorPrompt_StructuralResilience) verifying 7 compression-resilience properties

How to Test

# Build with restructured prompt
make build

# Run all tests (including new structural test)
make check

# Run just the structural test
go test ./internal/agentkit/ -run TestCoordinatorPrompt_StructuralResilience -v

The structural test verifies:

1. YAML frontmatter preserved (name, description, mode)
2. Identity-first opening contains "NEVER" + "reserve"
3. Section ordering: Critical Constraints < Protocol < Available Tools
4. All 6 behavioral rule markers present (comms_init, reserve, forge_review, hivemind_store, comms_inbox, forge_broadcast)
5. Every constraint line has uppercase RFC 2119 keyword
6. Explicit forge_review BEFORE forge_complete ordering
7. First 50% of body lines contain all critical constraints

How to Demo

Read the restructured internal/agentkit/content/agents/coordinator.md — the identity-first opening, Critical Constraints section, and numbered Protocol are self-evident. Compare against the previous version (flat bullet list) to see the structural improvement.

Key Files Changed

File	Change
internal/agentkit/content/agents/coordinator.md	Restructured from 22-line flat rules to 31-line compression-resilient format
internal/agentkit/agentkit_test.go	Added TestCoordinatorPrompt_StructuralResilience (151 lines)
openspec/changes/coordinator-prompt-hardening/	OpenSpec artifacts (proposal, design, specs, tasks)

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