`Paths` learns a new top-level filesystem dimension. Precedence in
descending order:
- workspace_id given (PR6+):
`{base_dir}/workspaces/{wid}/threads/{tid}/...`
`{base_dir}/workspaces/{wid}/users/{uid}/memory.json`
`{base_dir}/workspaces/{wid}/users/{uid}/agents/{name}/...`
- user_id only (legacy user-isolation): unchanged
- neither (very legacy): unchanged
API: every thread/sandbox/host/ACP helper now takes
`workspace_id: str | None = None` alongside the existing `user_id`,
plus `Paths.workspace_dir()` and `_validate_workspace_id()` for safe
id substitution. `resolve_virtual_path` likewise routes through the
new precedence. The migration script in T6.12 will walk the old
`users/{uid}/threads/...` tree and rewrite to the new shape.
15 new tests (workspace shape, validation, sandbox dirs, ACP,
user-memory/agents under workspace, traversal defence) pass; 46
existing path tests stay green.
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