feat: warn when a new worktree omits untracked nested git repos (meta-repos)

[diegoesolorzano]

Closes #4671

Problem

In meta-repo layouts (a parent repo containing independent nested git repos in subdirectories — e.g. frontend/, backend/ as their own repos, untracked by the parent), git worktree add only materializes files tracked by the parent. Orca creates the worktree without any hint, and the user lands in a tree with no application code. The auto-detected setup command (root lockfile install) reinforces the illusion of a complete checkout.

What this does

After a successful local create, if the source repo contains nested git repos not tracked by the parent, the create result carries a structured nestedRepos warning and the renderer shows a toast listing the directories (capped at 10, with the real remainder count), reusing the existing post-create toast pattern (localBaseRefRefresh). Creation itself is never blocked or failed — detection is best-effort and resolves null on any scan/git failure or timeout.

Tracked submodules never warn: candidates are filtered against staged gitlinks (git ls-files -z --stage, mode 160000) and .gitmodules declarations (git config --file .gitmodules) — those are intentional layouts that worktrees handle natively.

Implementation notes

• Reuses the existing scanner (scanNestedRepos from project-groups/nested-repo-discovery.ts) via its filesystem injection point, with two per-call overrides: isSelectedPathGitRepo: () => false (the scanner early-returns on git repos — it was built for non-git project folders) and readTextFile: async () => '' (gitignored nested repos are exactly the meta-repo case this warning exists for; the scanner's hardcoded skip-dirs and symlink safety are preserved). The default filesystem is extracted into an exported factory in a separate, behavior-preserving commit.
• Bounded work: depth ≤ 3, scanner cap 100, 5s timeout, exactly one ls-files + one config call regardless of candidate count. The scan runs concurrently with the create and is awaited just before the IPC result returns.
• Cross-platform: pure string relativization (handles both separators), slash-normalized pathspecs/display, WSL toplevel translated to UNC before the fs scan.
• Detection only runs for local git creates — remote (SSH) and folder-mode flows are untouched.

Tests

• 22 unit/integration cases for the detector (gitlink/.gitmodules exclusion, gitignore-override integration test on a real temp dir, caps/remainder, timeout/error paths, Windows/WSL path forms, subprocess count).
• IPC wiring cases in worktrees.test.ts (attach/absent/concurrency/rejection-safety/remote-not-invoked).
• Renderer slice cases (toast content, truncation remainder, absent field, coexistence with the base-ref toast).
• Scanner suite passes unchanged (the factory extraction is behavior-preserving).

X: @diegoesolorzano