chore(pre-commit): add check-merge-conflict hook

[lusoris]

Summary

• Adds no-conflict-markers local hook (scripts/ci/check-conflict-markers.sh) as a belt-and-suspenders companion to the existing check-merge-conflict entry (pre-commit-hooks v6.0.0).
• The new hook uses git grep (git's C binary, < 5ms) and fires on every commit regardless of file type; check-merge-conflict is Python-based and covers the standard three-way conflict syntax.
• PR fix(ci): remove committed merge-conflict markers from CI YAML files #174 (commit a4e9e707) fixed 70 files contaminated by conflict markers from commit 24bb5daf; this hook would have blocked that commit at the pre-commit gate.
• Cleaned up: PR was previously contaminated with 5,435 LOC of unrelated changes. Reconstructed as a clean 3-file, 55-line diff on top of master.

Type

• build / ci — tooling / infra

Checklist

• Commits follow Conventional Commits.
• make format && make lint is green locally (pre-commit hooks all passed on commit).
• Unit tests pass: meson test -C build.
• If I touched any SIMD/GPU code path, I ran /cross-backend-diff and the worst ULP is ≤ 2.
• If I touched a feature extractor with SIMD/GPU twins, I either updated every twin or listed the gap under "Known follow-ups" below.
• If I added a new .c / .cpp / .cu / .h / .hpp, it has the appropriate license header.
• If this is a breaking change, the commit message uses ! or BREAKING CHANGE:.
• If this PR adds an ADR, the ADR row lives in docs/adr/_index_fragments/<NNNN-slug>.md.

Bug-status hygiene (ADR-0165)

no state delta: tooling-only change; no bug opened or closed.

Netflix golden-data gate (ADR-0024)

• I did not modify any assertAlmostEqual(...) score in the Netflix golden Python tests.

Cross-backend numerical results

no rebase impact: pre-commit-only.

Deep-dive deliverables (ADR-0108)

• Research digest — no digest needed: trivial pre-commit hook addition
• Decision matrix — no alternatives: only-one-way fix
• AGENTS.md invariant note — no rebase-sensitive invariants
• Reproducer / smoke-test command — see Reproducer below
• CHANGELOG fragment — changelog.d/changed/pre-commit-conflict-marker.md
• Rebase note — no rebase impact: pre-commit-only change

Reproducer

# Verify the new hook catches a real conflict marker
echo "<<<<<<< HEAD" >> /tmp/test-conflict.txt
git add /tmp/test-conflict.txt 2>/dev/null || true
pre-commit run no-conflict-markers --files /tmp/test-conflict.txt
# Expected: hook fires and exits 1

# Verify clean commit passes
pre-commit run --files .pre-commit-config.yaml scripts/ci/check-conflict-markers.sh

Known follow-ups

None.

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