PR/change tracking now extracts and reports the source branch

What

• The code that scans bash command output for gh pr create / glab mr create invocations to link a session to a pull request was rewritten to also extract the target branch, from a --head/-H (gh) or --source-branch/-s (glab) flag, and validate it as a safe git ref before using it.
• The branch field reported to a host for VCS changes is now documented to carry this branch information more broadly: not just for providers without their own head-branch concept (like Gerrit), but for any provider when the change's kind is created, using the branch the create was opened from.

Why This lets a host display the branch a newly created pull request or change came from immediately, before the host's own lookup against the forge (like GitHub or GitLab) responds.