fix: allow deleting assistants and sessions whose workspace directory is unavailable

[legenddcr]

Fixes #3150

Summary

Two related fixes so items whose workspace directory is unavailable can still be deleted.

1. Assistant workspaces with legacy or missing directories (desktop)

delete_assistant_workspace, reset_assistant_workspace, and reset_workspace_persona_files rejected any assistant whose registry record still points at the legacy BitFun root ~/.bitfun/personal_assistant/workspace-<id> (directories the brand migration never relocated), and there was no path for records whose directory is already gone — the assistant could never be deleted from the UI.

• New WorkspaceService::is_manageable_assistant_workspace_path accepts both the current layout (~/.openbitfun/personal_assistant/workspace[-<id>]) and the legacy layout (~/.bitfun/personal_assistant/workspace[-<id>]). The legacy root name comes from the new legacy_hidden_data_directory() in core-types; the private constant in legacy-migration now reuses it.
• delete_assistant_workspace: managed paths delete the directory as before; unmanaged records whose directory no longer exists fall back to a registry-only cleanup (logged in English); existing directories outside both managed layouts are still refused — the deletion safety boundary is unchanged.
• reset_assistant_workspace and reset_workspace_persona_files accept both layouts.

2. Sessions whose workspace path is missing (web-ui)

FlowChatStore.deleteSession still required a workspace path before calling the backend, so sessions whose workspace directory is missing never reached the ID-first delete_session command; failures were swallowed and the session reappeared after reload.

• The guard is removed; the workspace id is snapshotted synchronously before the first await, pre-ID sessions fail loudly, and backend delete failures surface through the existing notification path after local cleanup.
• LocalSessionDriver cleanup moved into try/finally so local state is cleared even when backend cleanup fails.

Verification

• cargo test -p openbitfun-core --no-default-features --features agent-runtime,git --lib service::workspace::service::tests — 20 passed
• cargo test -p openbitfun-desktop --lib api::commands::assistant_workspace_delete_tests — 3 passed
• cargo test -p openbitfun-legacy-migration --test migration_engine_contracts — 10 passed
• cargo check -p openbitfun-desktop on this branch
• Front-end suites (FlowChatStore 146 passed, SessionModule 52 passed, session-drivers 14 passed) were run on the pre-rebase working tree; the touched front-end files are unchanged between that baseline and this branch's base, so the results carry over.

Remote scenarios

Deletion and reset are local registry + local filesystem operations invoked through existing Tauri commands; no remote-surface contract changed and no new commands were added.