Rebase session context

Active Rebase sessions now store turtlegit-session.json in the session's actual Git state directory, resolved with rev-parse --git-path. It records schema version 1, original branch/upstream/onto choices, Force, Preserve Merges, Fetch/Pull origin and configured auto-start. Optional fromLog distinguishes Log-origin Rebase, which has no configured completion buttons; older context without that field remains readable. Cherry Pick never records after-Fetch origin.

The backend writes context after Git returns an active session, including a conflict or Edit pause. This works without relying on the custom sequence editor being invoked. A successful operation with no remaining session creates no context file. The native window restores context when loading or refreshing an active session. Separate linked worktrees use their own Git state directory.

A reopened Fetch/Pull Rebase retains Show Log/Push/Send Mail/Rebase after successful completion, and restarting retains its upstream and configured auto-start. Branch/upstream/onto controls show the original choices rather than substituting Git's resolved onto hash for the upstream. Existing replay identity and source commit recovery are unaffected.

Missing, malformed or unknown-version metadata is ignored, leaving the existing Git-state fallback usable. Sessions created before this addition cannot recover an origin that was never recorded. Git removes the context along with its active state directory on completion or Abort; no repository config or user preferences are changed. The context does not persist a finished-session history report.

Core fixtures check recovered choices/origin, legacy and malformed fallback, successful cleanup, linked-worktree isolation and a Preserve Merges conflict that does not invoke the custom sequence editor. Native fixtures check a real Edit pause, a new controller reopening the session, restored branch/upstream/onto, completion command sets and restart. These use hidden views and injected handoffs; displayed focus, current screenshots and signed sandbox execution remain pending.