Add Linear round-trip fidelity integration test (#3187)

[harry-miller-trimble]

Summary

Implements the round-trip fidelity integration test requested in #3187.

What's included

TestLinearRoundTripCoreFields — Full push→pull integration test that:

1. Seeds 3 issues with varied priority (P0/P1/P3) and status (open/in_progress/closed)
2. Pushes to a stateful mock Linear GraphQL server
3. Pulls into a fresh database
4. Asserts title, description, priority, status, and external_ref all survive the round-trip

TestLinearRoundTripRelationships — Skipped spec test documenting that parent-child hierarchy, blocking dependencies, and issue type do not yet survive a round-trip (push doesn't send parent/relation/label fields). Becomes a regression gate when those features land.

Mock server design

The mockLinearServer is a stateful httptest.Server that:

• Only stores what the client actually sends (no fabricated fields)
• Routes by string match on GraphQL operations (no parser needed)
• Handles: issueCreate, issueUpdate, issues query, team states
• Assigns sequential identifiers (MOCK-1, MOCK-2, ...)

Key design decisions

• Engine-level testing (not CLI) — wires the same hooks as runLinearSync but avoids global state/flag noise
• Matches by external_ref — pulled IDs are hash-generated so they won't match source IDs
• Split into two tests — keeps assertions honest by only testing what the integration actually supports today

What's NOT asserted (documented gap)

Per the issue's assertion table, these fields are requested but push doesn't currently support them:

Property	Current status
Epic parent	Push doesn't send parentId
Dependencies	Push doesn't create relations
Issue type	Push doesn't send labels for type

The skipped spec test documents this gap and will automatically become a regression gate when those features are implemented.

Closes #3187

[codecov-commenter]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/audit/audit.go	40.00%	2 Missing and 1 partial ⚠️

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