fix(worktree): normalize Windows managed paths

[bobleer]

Summary

• convert Windows canonical managed-worktree roots from verbatim \\?\C:\... form to the compatible drive-letter form before constructing the Git target
• preserve canonical containment and symlink checks
• add a Windows-only regression asserting the target passed to Git never becomes //?/...

Fixes#2249

Root cause

tokio::fs::canonicalize returns a verbatim path on Windows. The managed-worktree flow then normalized backslashes for the Git CLI, turning \\?\C:\... into //?/C:/...; Git for Windows rejected that target while creating the linked worktree .git file.

Verification

• cargo test -p bitfun-core --no-default-features --features agent-runtime,git --lib service::worktree::tests:: (22 passed on macOS)
• pnpm run check:repo-hygiene
• git diff --check
• attempted a macOS-to-x86_64-pc-windows-msvc check; the host lacks the MSVC C headers required by libsqlite3-sys/libz-sys, so native Windows CI owns that platform run

Managed worktrees remain explicitly unsupported for remote workspaces, so this change was exercised only in the local-workspace path.

AI assistance

AI-assisted; locally tested as listed above, with the Windows-specific regression delegated to the native CI matrix.