fix: resolve working directory from primary mount on HostBackend

[RayCharlizard]

Summary

• On HostBackend, resolveWorkDir() falls back to os.homedir() for every Cowork session because the Electron app sends cwd=/sessions/{name} (a session-root guest path) and translateGuestPath() requires /sessions/{name}/mnt/{mount}/... — the session root never matches.
• This means Claude Code always starts in ~ instead of the user's project directory, breaking file access and context loading.
• The fix adds primary-mount derivation (mirroring BwrapBackend's existing logic) as a fallback before resorting to homedir.

Root Cause

translateGuestPath() uses a regex that requires a /mnt/{mount} component in the path. Session-root paths like /sessions/bold-sharp-clarke don't have one, so translation always fails. BwrapBackend avoids this by overriding spawn() with its own CWD derivation from the primary user mount. HostBackend goes through resolveWorkDir() which lacked equivalent logic.

Related: #389 (fixed a similar guest-path translation gap for CLAUDE_COWORK_MEMORY_PATH_OVERRIDE).

The Fix

When translateGuestPath() can't resolve a session-root CWD, derive the working directory from the primary user mount (first non-dotfile, non-uploads key in mountMap) — the same heuristic BwrapBackend uses. Falls back to homedir only when no user mount exists.

Verification

Tested with a Node.js harness simulating exact spawn parameters from live session logs:

Scenario	Before	After
Session root + user mount	~/ (bug)	/home/user/project/ (correct)
Session root + no user mount	~/	~/ (correct fallback)
Full guest mount path	Translated correctly	Unchanged
Host path passthrough	Passed through	Unchanged
sharedCwdPath precedence	Takes precedence	Unchanged

Test plan

• Start a Cowork session via Projects sidebar → verify Claude Code CWD is the project directory, not ~/
• Start a Cowork session via "New task" → verify same behavior
• Start a session with no folder selected → verify graceful fallback to homedir

🤖 Generated with Claude Code