…ct (#8164)

Co-Authored-By: Claude Opus 5.5 <[email protected]>

#8164)

A `.bare` layout root, whose `.git` file points at a repository inside it with
linked worktrees as siblings, is now rejected with
ProjectWorkspaceBareRepositoryError over WS, HTTP, and MCP. The guard rejects
only when a registered worktree lives under the proposed root, so a primary
checkout using `--separate-git-dir` inside itself with external worktrees still
works. The `gitdir:` file is parsed in one linear pass.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

Resolve conflicts by reporting the bare-root rejection as main's
ProjectInvalidError instead of a separate error class, and surface that
error's message over the WebSocket mutation path.

AI-assisted; verified with the tests and tools listed above.

…g index

A separate-git-dir checkout with linked worktrees inside it was rejected,
and a container reached through a symlinked path was accepted. Detect the
container by its git dir lacking an index, comparing real paths.

AI-assisted; verified with the tests and tools listed above.

A checkout made with git init --separate-git-dir has no index until its
first add, so a missing index alone rejected it. Also require linked
worktrees, which a checkout cannot have before its first commit.

AI-assisted; verified with the tests and tools listed above.