feat(workspaces): named groups of repos for federation

[spuentesp]

Summary

Implements the Lain Workspaces feature (spec at `docs/superpowers/specs/2026-08-12-lain-workspaces-design.md`, plan at `docs/superpowers/plans/2026-08-12-lain-workspaces.md`). A workspace is a named subset of repos from `repos.yaml` that the federation engine indexes as a coherent unit, switchable on server restart via `--workspace ` (or `--workspace auto` reading `~/.config/lain/active_workspace`).

What ships

• Workspace types + loader in `src/federation/workspace.rs`: `WorkspacesFile` (YAML), `WorkspaceSpec`, `WorkspaceSourceConfig`, `WorkspaceSource` trait, `WorkspaceDirSource` and `WorkspaceCloneSource` impls, `WorkspaceIndex`, `filter_repos_by_workspace` helper. 17 unit tests.
• ActiveWorkspace pointer at `~/.config/lain/active_workspace` (in `src/state.rs`) + `resolve_active_workspace` lookup helper. 6 unit tests.
• `load_federation_with_workspace` in `src/federation/loader.rs` + new `build_source_for` per-repo builder in `src/federation/config.rs`. Builds a workspace-aware federation the same way as `load_federation` but with a per-repo data subdir (so workspace restarts that swap the member set don't clobber state).
• `--workspace` flag wiring in `src/cmds/server.rs` + `LainServer::with_federation_and_workspaces` constructor in `src/server/mod.rs` so workspace MCP tools are registered.
• 4 new MCP tools in `src/mcp/federation_tools.rs`: `list_workspaces`, `get_active_workspace`, `get_workspace`, `get_workspace_graph` (the 4th gated on workspace being active).
• `lain workspaces` CLI subcommand in `src/cmds/workspaces.rs` with 10 actions: `create`, `add`, `remove`, `import`, `init`, `list`, `show`, `use`, `current`, `forget`.
• Federation dashboard (`src/mcp/federation_dashboard.html`): 3 new sections — active workspace panel (name + member table), config panel (paths + counts), per-workspace D3 force-directed graph view (with substring filter, color by repo_id, dashed lines for cross-repo Calls).
• `get_agent_strategy` in `src/tools.rs` gets a new "Workspace mode" section documenting the 4 new tools + the `repo_id` resolution rule when scoped.
• `GraphBackend` trait gets a new `all_edges()` method (for `get_workspace_graph` bulk-edge reads).
• `tests/workspace_e2e.rs` with 6 integration tests (2 pure-validation pass, 3 require `rust-analyzer`).
• `tests/e2e/workspace_e2e.sh` nightly e2e that exercises all 4 workspace tools + the workspace switch restart.
• `docs/FEDERATION.md` + `README.md` updated with a Workspaces section + pointer.

Test results

• 492/492 lib tests pass.
• `workspace_e2e.sh` passes all 5 assertions (list_workspaces, get_active_workspace, get_workspace for cross-workspace query, get_workspace_graph, and the ab→bc restart verification).
• `federation_e2e.sh` still passes (3 famous repos, no regression).
• 2/2 pure-validation `workspace_e2e.rs` tests pass.
• 4 `workspace_e2e.rs` tests + 3 `federation_integration.rs` tests are deferred to a working env (need `rust-analyzer` for `RepoIndex::index().await`; the same env-block documented in the test-gap fix).

Critical prerequisite

This PR cannot land before the test-gap fix's PR 1 (Pass A + Pass B in `project_repo`) merges — that was the explicit prerequisite captured in both the spec and the plan. The workspace's headline cross-repo `Calls` semantic depends on the test-gap fix producing those edges. PR 2 of workspaces (dashboard + e2e + docs + agent strategy) is safe to land independently.

Migration / rollout

`repos.yaml` schema unchanged. The federation tool set gains 4 new tools (`list_workspaces`, `get_active_workspace`, `get_workspace`, `get_workspace_graph`); the 6 existing tools are unchanged. Federation mode without a workspaces.yaml is identical to today — operators opt into workspaces by adding a file and passing `--workspace ` (or setting the active workspace pointer).

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