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

[zeroecco]

Summary

• Adds .githooks/commit-msg (validates ZIR-XXX: format, exempts Merge/Revert commits)
• Adds .githooks/prepare-commit-msg (auto-prepends ZIR-000: when prefix is missing; guards awk result before overwriting commit file)
• Adds scripts/setup-hooks.sh (one-command setup via git config core.hooksPath)
• Adds CONTRIBUTING.md documenting the convention and setup steps

Both hook files are tracked as mode 100755 (executable). No duplicate hooks/ directory. No conflicting setup scripts. No unreferenced template files.

Test plan

• Run bash scripts/setup-hooks.sh and verify git config core.hooksPath is set to .githooks
• Attempt a commit with a non-conforming message — confirm commit-msg hook rejects it
• Attempt a merge commit — confirm it is allowed through
• Attempt a commit without a ZIR prefix — confirm prepare-commit-msg prepends ZIR-000:
• Verify .githooks/commit-msg and .githooks/prepare-commit-msg are mode 100755 in git ls-files --stage

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