ZIR-000: Track commit message hooks and document commit format

[zeroecco]

Summary

• Add .githooks/commit-msg (mode 100755): validates ZIR-XXX: Description format; exempts Merge and Revert commits
• Add .githooks/prepare-commit-msg (mode 100755): auto-prepends ZIR-000: when prefix is missing, skips merge/squash sources
• Add scripts/setup-hooks.sh: one-command activation via git config core.hooksPath .githooks
• Add CONTRIBUTING.md: documents the commit format convention and hook setup steps

Changes from previous iteration

• Fixed dead awk variable: removed the unused -v first_line= argument from prepare-commit-msg; awk identifies the first matching line via !done without needing it passed in
• Single canonical hook directory: only .githooks/ exists; no duplicate hooks/ directory with non-executable copies
• Single setup script: only scripts/setup-hooks.sh using core.hooksPath; no conflicting setup-git-hooks.sh
• No unreferenced template files

Test plan

• Run ./scripts/setup-hooks.sh — should print confirmation, no errors
• Commit with ZIR-123: Some change — hook accepts it
• Commit with Some change — prepare-commit-msg prepends ZIR-000: Some change; commit-msg accepts the normalized form
• Commit with Bad message without running prepare hook — commit-msg rejects with clear error
• Merge commit passes without ZIR prefix
• Verify .githooks/commit-msg and .githooks/prepare-commit-msg are mode 100755 in git: git ls-files --stage .githooks/

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