Add missing unit test coverage for GithubCLI.merged_pr_by_head

[bjester]

Observed Behavior

GithubCLI.merged_pr_by_head() was introduced in src/treefort/gh.py to query merged PRs for branch pruning, but tests/test_gh.py only contains tests for pr_status and repo_status. merged_pr_by_head has zero unit test coverage.

Expected Behavior

GithubCLI.merged_pr_by_head() should have unit test coverage verifying command arguments, JSON parsing, --repo parameter inclusion, and exception handling.

Steps to Reproduce

1. Inspect tests/test_gh.py.
2. Observe only test_pr_status__calls_gh_and_parses_json, test_pr_status__with_owner_repo, and test_repo_status__calls_gh_and_parses_json are present.

Minimal Reproduction Environment / Conditions

• Version / Branch / Commit: release-v0.5.x@6d07c0c
• Component: tests/test_gh.py

Deliverables & Testing Requirements

• Add unit tests in tests/test_gh.py for merged_pr_by_head:
  • Standard call with head ref name, verifying --head, --state merged, and --json arguments.
  • Call with owner_repo parameter, verifying --repo argument.
  • Error condition handling when self.run raises CalledProcessError.
• Regression Safety: All tests must execute cleanly under pytest.

Technical Pointers & Root Cause Clues

• Suspected Components: src/treefort/gh.py:43-65, tests/test_gh.py
• Related Commits: 21941c3, bd7ed4c

Notes & Edge Cases

• Conforms to .aiassistant/rules/tests.md naming conventions.

Metadata

Severity / Priority: P2 (Medium)
Area: testing, gh

AI Usage

Drafted with Gemini following the DEEP framework based on code review findings against release-v0.5.x. Reproduction conditions and code references were verified in the codebase.