[P1][WS-01] Persistent workspace store and navigation

Programme: #1
Epic: #16

## Objective
Make the persistent workspace—not a conversation or tab—the durable user-facing unit of state.

## Scope
- Workspace/project home with stable IDs and addresses.
- Persist goal, source-backed object references, layouts, user overlays, saved filters, navigation history, files/artifacts and task links.
- Separate layout state from source data and execution/task state.
- Stable panel/view identity, back/forward navigation and restart restoration.
- Workspace archive/export hooks and redacted snapshot support.
- Local deterministic search/navigation over existing workspace state.

## Product rules
- Closing a chat must not remove the application the user created.
- Sorting/filtering/navigation must continue when inference is unavailable.
- Provider session/thread IDs remain optional references, never primary ownership.

## Acceptance criteria
- [ ] Create a workspace, change layout/filter state, close the conversation/app, restart and recover the same usable workspace.
- [ ] Workspace navigation works with inference disabled.
- [ ] Layout state can change without mutating source observations.
- [ ] Task/artifact/source links remain resolvable after restart and migrations.
- [ ] Archived/deleted workspaces follow retention and data-deletion rules.
- [ ] Stable workspace URLs/IDs support deep links from notifications/history.

## Dependencies
- CORE-02

**First phase:** P1
**Maturity target:** P2
**Owner:** workspace-ui-data

Start by reviewing the CORE-02 dependency and the workspace-ui-data area identified as the owner. Break the scope into persistence, navigation, layout separation, archive/export, and search before implementation. Done means the listed acceptance criteria pass, including restart recovery, inference-independent navigation, resolvable links, retention behavior, and stable deep links.

Written by the indexing model from the issue text.