feat: pre-flight validation before rebase operations

[dezren39]

Adds pre-flight checks before rebase operations (restack, sync, move) to detect conditions that would cause silent history corruption.

Changes

• New preflight module — shared check_all, check_single, report_and_check, summary
• Merge commit detection — git rev-list --merges blocks rebase unless --force
• Redundant branch detection — git cherry skips rebase when all commits already upstream
• Stale metadata detection — is_ancestor check catches out-of-band rebases
• ez restack --force — new flag to override preflight blocks
• ez move --force — new flag, replaces inline stale check with full preflight
• ez sync — preflight wired into restack phase with redundant skip + safe rebase mode
• Preflight summary receipts — JSON output for agent consumption

Commits (4)

• 6303758 feat: add pre-flight validation before rebase operations
• 7ddf9c1 feat: wire preflight into sync and move, add --force to move
• 8cd35b2 feat: emit preflight summary receipts from restack and sync
• 3673f32 test: add preflight report edge cases (empty summary, combined blocking, needs-restack-only)

Resolves rohoswagger#16

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Stack (this PR is 2 of 5):

1. feat/config (rohoswagger#9)
2. → feat/preflight ← you are here (dezren39#1)
3. feat/worktree-ops (dezren39#2)
4. feat/fork (dezren39#3)
5. feat/local-merge (dezren39#4)