fix(hooks): scope pre-commit clang-tidy to staged changes (#1264)

[Yyunozor]

Fixes #1264

Problem

scripts/hooks/pre-commit runs make -j3 -f Makefile.cbm lint
unconditionally on every commit. lint includes lint-tidy, which runs
clang-tidy over the whole LINT_SRCS tree. On a clean main checkout
that surfaces ~5,100 pre-existing error:-level findings across 113
files (mostly readability-magic-numbers), unrelated to whatever the
commit touches. The hook blocks every commit for every contributor with
clang-tidy on PATH — reproduced on a clean checkout, one-line non-C
change.

Fix

Add lint-tidy-diff (scripts/lint-tidy-diff.sh), a Makefile target
running the same clang-tidy binary/config, but passing clang-tidy's own
-line-filter so a diagnostic can only fire on lines the commit actually
added or modified (parsed from git diff --cached -U0 hunk headers).
Pre-existing findings on untouched lines are never reported, whether in
an untouched file or an untouched line of a touched file — baseline-aware
at the line level, not just the file level.

The hook now runs lint-ci (unchanged: cppcheck + clang-format +
no-suppress, already CI's contract) plus lint-tidy-diff instead of the
full lint target. make lint, make lint-tidy, and scripts/lint.sh
are untouched, still the full-tree audit; CI (_lint.yml →
scripts/lint.sh --ci) never ran clang-tidy and is unaffected. This
follows the scoping the issue itself suggested (remediation option 2), at
line rather than file granularity.

Verification

• Reproduced: clean checkout, git commit -s on a one-line
  CONTRIBUTING.md change, clang-tidy on PATH → blocked, 5,190
  pre-existing findings, commit did not happen.
• After the fix, same clang-tidy binary/config:
  • No staged .c changes → skipped, exit 0.
  • Clean addition to internal/cbm/cbm.c (one of the dirtiest files, 57
    pre-existing findings on a full-file run of the same edit) → exit 0,
    nothing reported.
  • A deliberate new finding (return 424242;) added to the same file →
    exit 2, reports only that one finding.
  • Header-only change (no paired .c change) → skipped, exit 0 (see
    Known limitation).
• scripts/lint.sh (cppcheck + clang-format + no-suppress) unaffected,
  still clean.

Known limitation

clang-tidy reports a header's diagnostics under whatever path it was
#include-d with, not reliably its working-tree path (verified:
absolute vs. search-path-relative, depending on include style). So
lint-tidy-diff scopes .c files only; a header-only commit isn't
covered and needs make lint-tidy. A proper diff-to-header mapping would
be a natural follow-up, left for the maintainer to size.

[DeusData]

Thank you for the contribution and for reproducing how repository-wide clang-tidy findings block unrelated staged changes. This is now triaged as a normal-priority contributor-workflow bug for 0.9.1-rc. Our community PR queue is currently quite full, so it may take a little time before we can complete the review and, if approved, merge it. We are doing our best to support community contributions and will return with code-grounded feedback as capacity opens.