Add Linear implementation consumer

[ferueda]

What changed

• Add the Linear implementation consumer as a thin durable Inngest vertical slice.
• Reuse the existing implementation agent, change-review/revision workflow, repository checkpoints, Grove cleanup, GitHub publication, and Linear projection primitives.
• Add route-specific implementation authority fingerprints so stale Linear changes cannot be silently reused.
• Add bounded review/revision behavior, exact checkpoint publication checks, failure recovery, and setup documentation.

Why

FER-326 needs an implementation path that can take an implementation-ready Linear issue through implementation, review, optional revision, publication, and Linear projection without introducing a generic workflow engine or persistence layer.

Verification

• make check
• 86 focused implementation/router/spec/triage tests
• Full test suite, lint, formatting, typecheck, build, and dist smoke pass

The unrelated local skills-lock.json and .agents/skills/dockerfile-validator/ changes were intentionally not included.