hook: doctor gate blocks commits on bloat in files that aren't part of the commit #357

Follow-up from the review of #356 (#320). Operator-approved as the step after the #318 series.

Since #356, the pre-commit hook tidies only staged vendor files and never sweeps unrelated work. In repos without a verify:adf script, the hook's evidence gate (charter doctor --adf-only --ci) still scans the whole working tree for vendor-file bloat. So a commit can be blocked by state that isn't part of it. In review, 5 of 13 scenarios newly blocked vs. the old hook, which only passed them by rewriting or sweeping files the user hadn't staged:

• an unstaged bloated GEMINI.md while committing something else
• a partially staged vendor file, which the hook skips by design (CP-3)
• a planned module that has unstaged or untracked work, which the hook skips by design (CP-4)

#356 keeps these blocks and explains them (option A). This issue does the rest:

• B: in hook context, scope doctor's bloat checks to the staged vendor files.
• C: in hook context, bloat in files the tidy block deliberately skipped does not block the commit. CI's doctor --ci still enforces it.

That makes CP-3/CP-4 truly non-blocking in the default hook. Option D (doctor reads the staged blob) is the exact version, if B+C prove too coarse. Review analysis: #356 review comment.