Cleanup blocks on branches merged into non-main parent branches

[acreeger]

Problem

When a branch's PR was merged into a non-main parent branch (e.g., a phase1 branch), il cleanup blocks with:

❌ Cannot cleanup:

Branch 'feat/issue-ai-44__preserve-chat-session' has not been pushed to remote and is not merged into 'develop'.
Deleting this branch would result in data loss.

Root Cause

The safety check flow in ResourceCleanup.ts (line ~950):

1. getMergeTargetBranch() checks for parentLoom.branchName in metadata. If none, falls back to configured mainBranch (e.g., develop).
2. GitHub deleted the remote branch after merge → remoteStatus.exists is false.
3. Falls to isBranchMergedIntoMain(branch, mainBranch) which checks git merge-base --is-ancestor against develop.
4. Branch was merged into a parent branch (not develop), so the check fails → blocked.

Workaround

il cleanup <identifier> --force bypasses the safety check.

Suggested Fix

When the remote branch doesn't exist and the branch isn't merged into the configured main branch, check via the VCS provider if a merged PR exists for this branch. If a merged PR exists, the work is safe — no data loss risk.

Relevant code:

• src/lib/ResourceCleanup.ts:914-969 — safety check logic
• src/utils/git.ts:1076-1100 — getMergeTargetBranch()
• src/utils/git.ts:858-870 — isBranchMergedIntoMain()