fix(worktrees): remove worktrees in the background (#3319)

* refactor(worktrees): fetch source once during creation

* fix(worktrees): remove worktrees in background

* fix(worktrees): show background removal progress

* fix(worktrees): name the worktree in removal toasts

packages/web/server/lib/git/DOCUMENTATION.md

@@ -49,7 +49,7 @@ The following functions are exported and used by the web server:
 ### Worktree Operations
 - `getWorktrees(directory)`: List all git worktrees for a repository.
 - `validateWorktreeCreate(directory, input)`: Validate worktree creation parameters (mode, branchName, startRef, upstream config).
-- `createWorktree(directory, input)`: Create a new worktree (supports 'new' and 'existing' modes, upstream setup). After populating the worktree, the repository's `post-checkout` hook runs once with git's standard arguments (null ref as previous HEAD, the checked-out HEAD, and flag `1`) from the worktree directory, mirroring `git worktree add` without `--no-checkout`; a missing or non-executable hook is skipped and a failing hook is logged as a warning, never failing worktree creation or the session bootstrap.
+- `createWorktree(directory, input)`: Create a new worktree (supports 'new' and 'existing' modes, upstream setup). When the current tracked branch has no unpublished commits, the UI supplies its remote-tracking ref and this operation fetches that branch once before creating the worktree. A failed fetch falls back to the local branch and reports `sourceFetchFailed`; other remote start refs still require an existing local ref when their fetch fails. After populating the worktree, the repository's `post-checkout` hook runs once with git's standard arguments (null ref as previous HEAD, the checked-out HEAD, and flag `1`) from the worktree directory, mirroring `git worktree add` without `--no-checkout`; a missing or non-executable hook is skipped and a failing hook is logged as a warning, never failing worktree creation or the session bootstrap.
 - `removeWorktree(directory, input)`: Remove a worktree (optionally delete local branch).
 - `isLinkedWorktree(directory)`: Check if directory is a linked worktree (not primary).
 
@@ -143,6 +143,7 @@ The following functions are internal helpers used by exported functions:
 - `path`: Absolute path to worktree directory.
 - `directoryCreated`: Present when create returned after the target directory exists while background Git/bootstrap work continues.
 - `bootstrapStatus`: Background setup state. The legacy `status` remains `pending`, `ready`, or `failed`, while `phase` reports `directory-created`, `git-ready`, or `setup-ready`. Fast create starts at `pending`/`directory-created`; population and upstream Git completion advances to `pending`/`git-ready` before setup/start scripts; completed setup is `ready`/`setup-ready`. A missing in-memory state falls back to `ready`/`setup-ready`; clients continue to accept legacy status responses that omit `phase`.
+- `sourceFetchFailed`: Present when the automatic source-branch fetch failed and creation fell back to the tracked local branch.
 - Fast-create background failures remove OpenCode sandbox metadata for directories that never became Git worktrees, and remove the pre-created directory only if it is still empty. User-created files are never recursively deleted by this cleanup.
 - Worktree removal waits for any active create/bootstrap task for that directory before deleting it, preventing a background Git or setup task from restoring removed state or racing filesystem cleanup.
 - Worktree bootstrap retries transient `index.lock` conflicts. If the lock remains byte-for-byte and metadata-identical across the retry window, it is treated as stale, removed, and population continues automatically; changing locks are left untouched and reported as failures.