feat(vcs): open the existing worktree that already holds a branch

[enisze]

Summary

Reopening a branch through a worktree thread previously failed with an opaque git worktree add failed whenever git refused the add. This PR handles the cases the caller cannot fix by hand and resolves GitHub account routes through symlinks.

VCS worktree handling (GitVcsDriverCore.ts)

• Branch already checked out in another worktree → hand that worktree (and the threads rooted at it) back instead of failing. When the claim is the main repo checkout, explain clearly that it cannot also become a worktree.
• Derived directory is taken (a worktree kept its directory after its branch was switched) → fall back to a sibling <name>-2.
• Stale worktree whose directory was removed behind git's back → git worktree prune and retry once.
• All of this runs on the failure path only, so a successful add stays a single spawn.

GitHub account resolution (GitHubAccountResolver.ts)

• Resolve account routes through symlinks: a cwd and the stored project root routinely name one directory via different symlinks (/tmp vs /private/tmp, symlinked home/worktree dirs). On a literal miss, resolve both sides' real paths (cached, TTL) before falling back to the ambient account.
• Sharpen the token-failure warnings to reflect that commands now refuse rather than silently act as the ambient account.

Web (ChatView.tsx)

• Preserve the previous draft when opening a review chat beside the current one, so a fresh review draft stays attached to its worktree.

Test plan

• New GitVcsDriverCore.test.ts cases: reuse a worktree that already holds the branch, explain a branch only the requesting checkout holds, create a sibling worktree when the directory is taken, reopen a branch whose worktree directory was deleted.
• New GitHubAccountResolver.test.ts case: resolve a cwd that reaches the workspace root through a symlink.

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