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'aicodeman': patch
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Fix two multi-user scoping holes in the new filesystem path picker. `GET /api/filesystem/browse` and `GET /api/filesystem/preview` accept an optional `sessionId` that contributes the session's working directory as a browse root, but they resolved it straight off the session map without an ownership check, unlike the nine other session-scoped handlers in the same route file. A non-admin could therefore pin another user's working directory as a root simply by passing their session id, then list and preview files under it. Both endpoints now run `canAccessOwned` and report 404, which also avoids confirming that a session id exists.

Separately, `Home` and `CASES_DIR` were unconditional browse roots for every caller. Per-user spaces live at `<USER_SPACES_DIR>/<username>`, which is inside `homedir()`, so the `Home` root alone exposed every other user's workspace to any authenticated user. In multi-user mode a non-admin now gets only their own space plus anything explicitly listed in `CODEMAN_FILE_PICKER_ROOTS`; `/mnt/d` is no longer offered by default, since a broad host mount should be an explicit operator decision in a multi-user deployment. Admins keep the host-wide roots, and single-user mode is unchanged.

Both holes are regression-guarded in `test/routes/file-routes.test.ts`, verified to fail against the previous code. Multi-user mode is opt-in and off by default, so single-user installs were never affected.