The test used 'github.com/test/extension' as the git source, but
parseGitUrl() requires a 'git:' prefix for bare hostnames. Changed
to 'git:github.com/test/extension' so the source is correctly
parsed as a git type and update() actually runs.
- Store previousSessionFile before creating branched session
- Update sessionFile after writing new branch file
- Pass parentSession when forking from first message
- Add --local to git config in tests to prevent repo escape
Tests used setExtensionPaths but update() iterates packages, not
extensions. All 5 non-pinned tests were silently passing as no-ops.
Also add regression test for scope-aware update behavior.
