Fix worktree base branch updates for active draft

[juliusmarminge]

• Route branch selection to the active draft session
• Prevent draft branch sync from overwriting an in-progress worktree setup
• Add coverage for the draft-specific branch selector flow

What Changed

Why

UI Changes

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes

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Note

Medium Risk
Touches branch/worktree synchronization logic for draft threads; mistakes could cause incorrect branch context or unexpected worktree setup behavior, but changes are localized and covered by new tests.

Overview
Fixes draft worktree routing so branch/base-branch changes act on the active draft route session when a draftId is present, instead of resolving drafts only by threadRef (applies to BranchToolbar, BranchToolbarBranchSelector, and GitActionsControl).

Threads draftId from ChatView → ChatHeader → GitActionsControl, adds an isSelectingWorktreeBase guard to prevent automatic branch sync from overwriting user selection while a worktree draft is being configured, and adds browser tests covering the draft-specific branch selector flow and the non-overwrite behavior.

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Note

Fix base branch updates for active worktree draft session in branch toolbar

• BranchToolbar, BranchToolbarBranchSelector, GitActionsControl, and ChatHeader now accept a draftId prop and use getDraftSession(draftId) to scope branch changes to the active draft route session rather than the thread-scoped draft.
• GitActionsControl suppresses automatic branch syncing when a worktree draft is being configured (no server thread, no worktreePath), preventing a preselected base branch from being overwritten.
• Live branch sync now falls back to the draft session's branch when no active server thread is present.
• A new browser test in ChatView.browser.tsx verifies that branch changes apply only to the active draft session and that stale sessions are unaffected.

^{Macroscope summarized 8490aff.}

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[macroscopeapp]

Approvability

A prior Macroscope approval was dismissed. Re-evaluating efce830…

No code changes detected at 8490aff. Prior analysis still applies.

Dismissing prior approval to re-evaluate ace12af

Dismissing prior approval to re-evaluate efce830