Add Linear PR closeout guardrails

[cjarguello]

Summary

Adds lightweight PR-to-Linear closeout guardrails to the active Linear process docs so merge closeout, retroactive linking, project scope, status class, and label hygiene are checked consistently instead of handled ad hoc.

Linear

BIT-357 — Add PR-to-Linear closeout guardrails to Linear process

• PR-to-Linear mapping verified when applicable
• Project scope / status class / labels verified when applicable
• Bidirectional PR + Linear links verified when applicable

Taylor01 Portability Check

• T01_LAYER: policy
• T01_SPECIFICITY: bitpod-specific
• T01_COUPLING: Linear process docs remain BitPod-specific; no runtime adapter changes
• T01_ACTION: keep-local
• T01_BYPASS: none
• T01_BYPASS_REASON: n/a
• T01_REVIEW_TRIGGER: revisit if the Linear process is extracted into a reusable Taylor01 adapter

Validation

• git diff --check
• Docs-only change

Risk & Rollback

• Risk level: Low; process docs and PR template only.
• Rollback plan: revert this PR and remove the 2026-04-28 maintenance update from the Linear operating guide/changelog.