fix: add merge-commit guard and diagnostic warnings to get_pr_commit_messages()

[Copilot]

get_pr_commit_messages() used a bare except Exception: pass, silently swallowing all failures from git log HEAD^1..HEAD^2 — including cases where HEAD isn't a merge commit (shallow fetch, non-merge checkout), making failures completely invisible.

Changes

• Merge-commit guard: runs git rev-parse --verify HEAD^2 before the range query; emits a ::warning:: annotation with stderr details and returns early if HEAD lacks a second parent
• git log failure logging: logs result.stderr as a ::warning:: annotation when the command exits non-zero, instead of silently falling through
• Exception logging: both except Exception blocks now emit ::warning:: annotations rather than silently passing

# Before
try:
    result = subprocess.run(["git", "log", ...], check=False)
    if result.returncode == 0 and result.stdout:
        return [...]
except Exception:
    pass          # ← silent, no diagnostic output
return []

# After
# 1. Guard: verify HEAD^2 exists and warn if not
# 2. Run git log; warn with stderr on non-zero exit
# 3. Catch/log any subprocess exception

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