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

[orioltf]

What to build

Make the changed-file analyser content-aware so comment-analyzer spawns whenever the diff adds or removes comment lines, not only when a doc file (.md/.mdx/docs/) changes. This is the tracer-bullet slice: it establishes the diff → analyser plumbing that the errors and types gates reuse.

End-to-end behaviour: the orchestrator passes diff content (alongside the changed-files list) into the analyser; the comment gate inspects added/removed (+/-) lines for comment tokens (//, /* … */, #, <!-- -->, JSDoc *), excluding the SPDX/license header so boilerplate doesn't trip it. Per ADR-0008 (amended, #212), bias toward spawning on ambiguity — a false-positive spawn is a cheap empty result block; a false-negative is the PR #5612 miss this fixes. The existing doc-file trigger stays (a Markdown/doc change still spawns it unconditionally).

Acceptance criteria

• A code-only diff that adds/removes inline comments spawns comment-analyzer (the PR #5612 case: a comment change in a .tsx now spawns it).
• A diff with no comment changes and no doc files does NOT spawn comment-analyzer.
• Doc-file changes (.md/.mdx/docs/) still spawn it unconditionally.
• The comment gate is a pure function over diff hunks with node:test fixtures: added comment, removed comment, no-comment source edit, doc-file, SPDX-header-only.
• The analyser↔orchestrator contract carries diff content; existing path-based gates are unaffected and their tests still pass.

Blocked by

• docs(unic-pr-review): ratify spawn-gating parity contract (ADR-0008 amendment + PRD elevation) #212