The `ReviewBranchDiff`, `ResolveConflictsWithAgent`, and
`ResolveConflictedFilesWithAgent` actions are dispatched from the git UI
(`git_ui/src/project_diff.rs` and `git_ui/src/conflict_view.rs`), not
the agent panel. Their `source` field in the "Agent Thread Started"
telemetry event was set to `"agent_panel"` — this changes it to
`"git_panel"`.

Also threads the `source` parameter through `activate_draft` and
`ensure_draft` so that drafts created from the sidebar correctly report
`source = "sidebar"` instead of `"agent_panel"`.

Release Notes:

- N/A