[FEATURE] Coder gate: surface a GO that changes no functional production code (docs/comment-only fixes)

[Defilan]

Problem

A GO whose committed diff is docs / comments / tests only produces a hollow GATE-PASS: the bite check has no production code to revert, so it is vacuous, and prose (docs or Go comments) can claim behavior that is not implemented.

Observed (dogfood #850): a bug-labeled issue ("hostpath PVC blocked on tainted GPU nodes") produced a docs + comment-only change. The added comment claimed "the operator surfaces a clear warning when a PVC source is used on a tainted node and the PVC has not yet bound" — but no warning code exists in the diff. It GATE-PASSed, because there was no Go logic to fail and no biting test.

Proposed fix

A non-blocking coder-gate check that flags when a GO's committed diff has no functional production change (every changed .go line is a comment/blank; non-.go changes are docs/markdown). Record it on the result (e.g. noFunctionalChange: true) and log it, especially for bug-labeled issues where a fix that changes no code is a red flag. Surfaces for reviewer/human judgment; does not change the verdict.

This is the deterministic half. The semantic half (prose claims an unimplemented feature) remains a reviewer-layer concern, but surfacing "this GO changed no code" removes the misleading clean GATE-PASS.

Acceptance

• A GO whose diff is docs/comments/tests only records noFunctionalChange: true.
• A GO with real production logic does not.
• Non-blocking (verdict unchanged); visible in status extra + logs.