feat(unic-pr-review): content-aware errors gate for silent-failure-hunter

[orioltf]

What to build

Replace silent-failure-hunter's "any non-test source file" path trigger with a content gate that spawns when the diff adds or removes error-handling constructs — try/catch/finally, throw, .catch(, Promise reject, error/err identifiers in handler positions. Reuses the diff → analyser plumbing from #213. Bias toward spawning on ambiguity (per ADR-0008, #212).

This is the weakest-fidelity gate — "error handling changed" is genuinely semantic and hard to capture with deterministic token matching. The slice must mark it (in code comment + ADR note) as the first candidate to promote to model judgment (Y-llm) if it proves too noisy or too blind in practice.

Acceptance criteria

• silent-failure-hunter spawns when the diff touches error-handling constructs.
• It does NOT spawn for a pure non-error source edit (e.g. a rename or a numeric-constant change).
• Ambiguous hunks bias toward spawning rather than skipping.
• The gate is a pure function over diff hunks with node:test fixtures: try/catch added, .catch() removed, bare throw, no-error-handling source edit.
• A code comment and ADR-0008 note record it as the Y-llm-promotion candidate.

Blocked by

• feat(unic-pr-review): content-aware comment gate + diff-to-analyser plumbing #213