test(mcp): enforce directional tool contract

[barbatdev]

🔗 Linked Issue

Upstream issue: Gentleman-Programming#717 (status:approved).

This is a fork-first staging PR and intentionally does not close the upstream issue.

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🏷️ PR Type

• type:bug — Bug fix
• type:feature — New feature
• type:docs — Documentation only
• type:refactor — Code refactoring (no behavior change)
• type:chore — Maintenance, dependencies, tooling
• type:breaking-change — Breaking change

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📝 Summary

• Replace conservative exact live equality with bounded baseline-to-live directional compatibility.
• Add deterministic drift diagnostics and a build-tagged, explicitly gated fixture writer.
• Remove duplicate broad registry inventories while retaining focused behavior and profile checks.

📂 Changes

File	Change
internal/mcp/tool_contract_test.go	Adds directional comparison, widening rules, deterministic drift records, and fixture self-canonicality validation.
internal/mcp/tool_contract_update_test.go	Adds the build-tagged writer with CI, mode, overwrite, compatibility, temporary-file, and rename safeguards.
internal/mcp/mcp_test.go	Replaces duplicate full inventories with dynamic comparisons while preserving focused assertions.

🧪 Test Plan

• Compatible live-only simple additions pass; narrowing and unproven changes fail closed.
• Focused contract and writer-refusal tests pass.
• go test ./internal/mcp -count=1 passes with 88.8% package coverage.
• go build ./... and go test -tags e2e ./internal/server/... pass.
• Ordinary builds exclude writer behavior and preserve fixture SHA-256.
• go test ./... is fully green locally: two known macOS /var versus /private/var assertions fail outside internal/mcp.

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🤖 Automated Checks

Fork-first note: the upstream issue is approved, but the fork-local governance job may not resolve an upstream issue through context.repo. The final upstream PR will use the canonical issue-closing reference.

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✅ Contributor Checklist

• The approved upstream issue is linked above.
• Exactly one PR type and matching label are present.
• Commits follow conventional-commit format.
• No Co-Authored-By trailers are present.
• The exact delta is 286 additions plus 67 deletions across three paths.
• The fixture delta is zero and runtime behavior is unchanged.

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💬 Notes for Reviewers

Fork-first stack, PR 3 of 3. PR 1 and PR 2 are already integrated into the fork base, so this PR shows only the 353-line directional/writer/inventory delta.

The complete integrated feature passed SDD verification for 9/9 requirements and 16/16 scenarios. Native high-risk RDD reviewed all four integrated MCP paths through risk, resilience, readability, and reliability lenses and approved the candidate without a correction transition. Four advisory-only findings remain documented for later work.