Adds ship provenance detection and emission in execute_bash_async():
- Detects git push to main/master commands
- Captures current branch, HEAD commit, git user as actor
- Emits ship.prepared event with ShipProvenance payload
- Logs to stderr as interim routing (event stream integration pending)
This is the first wired provenance event — schema (§4.44.5) now has
runtime emission at actual git operation boundary.
Verified: cargo build --workspace passes.
Next: wire ship.commits_selected, ship.merged, ship.pushed_main events.
Refs: §4.44.5.1, ROADMAP #4.44.5
12f1f9a74e