Expose merge and rebase operation state

[cosentinode]

Summary

• add typed merge/rebase operation state to Git worktree status
• detect MERGE_HEAD plus rebase-merge and rebase-apply independently of conflict entries
• retain cherry-pick/revert guardrails and show an in-progress banner in the status panel

Verification

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace

Fixes #53

[cosentinode]

rebase-apply is not sufficient to identify a rebase: a paused git am session creates this directory too (with rebase-apply/applying). I reproduced this with an empty patch: git status reported "You are in the middle of an am session", while this branch would set status.operation to RepositoryOperation::Rebase and show REBASE IN PROGRESS. That makes the newly exposed typed state false and will also misdirect later recovery UX. Please distinguish the git am form (or model it separately) while retaining its existing guardrail. The test currently creates a bare rebase-apply directory, so it misses the real marker layout.

[cosentinode]

Fixed the paused git am misclassification in commit 28bde7a.

rebase-apply/applying is now excluded from typed rebase state while remaining covered by the clean-worktree guardrail as an am operation. The regression creates a real paused empty-patch git am session and verifies that status reports no rebase and checkout remains blocked.

Verification:

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace

[cosentinode]

ZERO CONCERNS

Reviewed current head 28bde7a25e4ef2d197240ebc6866b7b3cfbe97a5 against #53, including the complete diff, surrounding Git/TUI code, operation-state edge cases, added regression coverage, and CI. Independent verification passed: cargo fmt --all --check, cargo clippy --locked --workspace --all-targets -- -D warnings, cargo test --locked --workspace, and git diff --check origin/develop...HEAD. GitHub Rust and GitGuardian checks are passing.