fix(git): merge-aware commit-clean for MERGE_HEAD lineage

[diazMelgarejo]

Sync merge-aware commit-clean.sh from orama canonical guard scripts.

• Auto-detect MERGE_HEAD during in-progress merges (no --merge flag).
• Retain full parent lineage for two-parent and N-way octopus merge commits.
• Clear merge state files after successful commit.
• Add commit_clean_test.sh regression coverage.

Required for history-preserving synthesis runbooks that use commit-clean after git merge --no-commit.

  

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