fix(gardener/classifiers): raise DIFF_CAP and filter noise

[serenakeyitan]

Summary

• Raise DIFF_CAP from 20KB → 200KB in both anthropic.ts and claude-cli.ts — 20KB is ~6% of Haiku 4.5's 200K window and truncates typical feature PRs at ~300-400 lines.
• Raise DIGEST_BUDGET_BYTES from 30KB → 100KB in tree-digest.ts.
• Extract DIFF_NOISE_PATTERNS from sync.ts into a new shared engine/classifiers/diff-filter.ts. Both classifiers now strip lockfiles, dist/build/out/coverage/node_modules/__pycache__ hunks, and minified/map/snap artifacts BEFORE applying the byte cap, so the cap bounds real code instead of noise.
• Skip the entire ## Diff section when filtering leaves nothing (e.g. lockfile-only PRs now get a clean prompt instead of an empty code fence).

Total prompt input rises from ~55KB to ~310KB (~38% of Haiku's 200K window), leaving headroom for prompt-caching stability, tokenizer variance on mixed Chinese/code content, and Anthropic's soft ~180-190K prompt-length threshold.

Refs #338.

Test plan

• pnpm typecheck clean
• pnpm test — 1203 passed, 0 failed, 51 skipped (no regressions)
• New tests/gardener/gardener-diff-filter.test.ts — 11 cases covering lockfile drop, dist/ drop, minified artifacts, all-noise PRs (empty output), fail-open on malformed hunks, and real-code-only preservation
• Manual: run first-tree gardener comment --pr N --repo o/r on a lockfile-heavy PR locally and confirm the verdict is grounded in real code, not lock diff

[yuezengwu]

Reviewed the diff and ran the new tests locally (11/11 passing, typecheck clean). The fix is well-scoped: filtering noise before the byte cap is the right ordering, the ~38% of Haiku's 200K window for total prompt input leaves sane headroom for prompt-caching + tokenizer variance, and the new filterDiffNoise tests cover the important shapes (lockfiles, dist hunks, all-noise → empty, fail-open on malformed headers, real-code preservation). Guarding the ## Diff section so an all-noise PR gets no empty code fence is a nice touch.

One thing that does not match the PR description, worth a small follow-up:

• The description says "Extract DIFF_NOISE_PATTERNS from sync.ts into a new shared engine/classifiers/diff-filter.ts", but src/products/gardener/engine/sync.ts (lines 678–686) still defines its own private DIFF_NOISE_PATTERNS and isDiffNoise. The two lists are currently identical, but there are now two sources of truth to keep in sync. formatPrDiffForPrompt operates on PrFileChange[] (file-level) while the new helper operates on raw unified-diff text, so they can't share the whole function — but sync.ts could still import { DIFF_NOISE_PATTERNS, isDiffNoise } from "./classifiers/diff-filter.js" and drop its local copy. Not a blocker, but worth closing the loop so the next pattern tweak doesn't have to be made in two files.

Minor/optional observations:

• The b/(.+)$ header regex will misbehave on filenames containing b/ (e.g. a path like foo b/bar.ts). Extremely unlikely in real repos; fail-open means the worst case is "noise slips through," so not worth fixing unless you want belt-and-suspenders.
• The split on /(?=^diff --git )/m assumes that literal string never appears at the start of a line inside a patch body. Fine for gh pr diff output in practice; just noting for anyone who runs into a diff-of-a-diff edge case later.

Approving — the duplication note is the only thing I'd actually want addressed, and a follow-up PR is fine if you prefer to keep this one tight.

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