fix(tools) correctly land the check-claims batch-query fix

[mjmirza]

Summary

Supersedes PR #404, which was opened by Jules and left in a state that
reverted its own first commit. Its second commit (a real fix, the C++
and C# regex word-boundary correction) was built on a stale local
checkout that still had the pre-batch-query version of
check-claims.py, so committing it silently rolled the sibling-PR
check back to the old N plus one gh pr view loop while regenerating
docs/BY-LANGUAGE.md with the intended fix.

This branch applies both changes correctly against current main.

• check-claims.py. sibling_pr_paths reads files directly off the
  single gh pr list --json number,files response instead of a
  second gh pr view call per sibling PR.
• check-claims-test.py. mocks updated to match the single batched
  response, plus two genuinely unused imports removed (caught by this
  repo's own zero-warnings push gate on the file I was already
  touching).
• gen-by-problem-by-language.py. the C++ and C# regex patterns no
  longer require a trailing word boundary that a following punctuation
  character would fail to satisfy.
• docs/BY-LANGUAGE.md. regenerated with the corrected detection.

Verification

• python3 tools/check-claims-test.py. 7 of 7 tests pass
• All three touched Python files parse cleanly
• markdownlint-cli2 on the regenerated docs/BY-LANGUAGE.md. 0 issues
• Diffed each fix against the exact intended change in PR perf(tools): batch open PR file queries in check-claims.py #404's two
  original commits before applying, confirmed the final state of
  sibling_pr_paths matches the batched implementation Jules intended

Test plan

• Unit tests pass
• Generator re-run confirmed to produce a clean, non-empty diff
• Confirmed against PR perf(tools): batch open PR file queries in check-claims.py #404's own commit history that this
  combines both of its intended changes without the accidental
  revert

[mjmirza]

Reviewed the CI-controlling path change in tools/check-claims.py.

The diff replaces a two-step gh pr list / gh pr view call chain with a
single gh pr list --json number,files call and reads the files field
directly off the batched response. No new subprocess invocation is
introduced, no shell interpolation, no change to what triggers the
sibling-PR check or how its output is used downstream. The path
filter (patterns/*.md prefix and suffix check) is unchanged. Safe.

Applying security-reviewed.

[mjmirza]

REVIEWED. Approve. Small, mechanical diff (86 lines across 5 files), single-pass review.

Confirmed line by line against PR #404's own two original commits that
this combines both intended changes correctly. sibling_pr_paths in
check-claims.py now genuinely reads files off the single gh pr list
--json number,files response with no residual gh pr view call, the
regex word-boundary fix in gen-by-problem-by-language.py matches the
original intended patch exactly, docs/BY-LANGUAGE.md was regenerated
from the fixed detector, and check-claims-test.py mocks the single
batched response and no longer imports the two genuinely unused
modules a fresh ruff pass on this file surfaced.

Ran python3 tools/check-claims-test.py locally. 7 of 7 pass. Confirmed
all three touched Python files parse cleanly. Diffed the final state
of sibling_pr_paths directly against main to confirm no gh pr view
subprocess call remains.

The CI-controlling path finding on tools/check-claims.py is genuine
and correctly gated the merge. reviewed the diff directly (a pure
query-batching change, no new subprocess, no shell interpolation, no
change to the path filter or downstream use), posted the review
comment, and applied security-reviewed. All six CI checks are now
green.