feat: Space layer above Project + RunContext + Single Agent CDP

[4pmtong]

Related Issue

#1651

Description

Introduces the Space layer above Project: a Space (folder-backed or blank)
holds Projects; a Project is one conversation; a Run is one execution.

Three load-bearing pieces ship together:

1. Space -> Project -> Run data model — server tables + migration + Brain
   workspace state + frontend stores + UI.
2. RunContext per-Run isolation replaces ad-hoc os.environ writes in
   chat_controller and toolkits; concurrent chats no longer overwrite each
   other's working dirs, API keys, browser ports, or CAMEL log paths.
3. Single Agent gains a CDP browser (shared pool with Browser Agent +
   try/finally release).

Plus: durable cross-restart project_context for follow-up turns, 4-layer
Project display-name backfill, camel_logs hidden from agent file UI, Brain
auth scaffold (NoneAuth default — local desktop Brain only),
file_save_path keyed by canonical user_id with project-base-level legacy
email fallback.

Deployment — required before merging

• Run preflight (server/scripts/space_migration_preflight.py) to catch
  cross-user project_id collisions before the migration silently rewrites
  them.
• Schedule a maintenance window: Postgres-only migration, in-tx UPDATE
  on chat_history holds row locks for the duration.
• Rollback = pg_dump snapshot restore, not alembic downgrade — the
  downgrade drops new shape but does not revert in-place data backfills.
• Single worker / single replica — per-Space write locks in
  apply_service.py + space_overlay_client.py are process-local.
• workspace_controller is gated by manifest.get("deployment") == "local" — do not enable on any cloud / multi-tenant Brain until a real
  IAuthProvider lands.
• ~/eigent/{email_sanitized}/ legacy artifacts stay readable via fallback;
  old directories are not auto-migrated.

Testing Evidence (REQUIRED)

• I have included human-verified testing evidence in this PR.
• This PR includes frontend/UI changes, and I attached screenshot(s) or screen recording(s).
• No frontend/UI changes in this PR.

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Contribution Guidelines Acknowledgement

• I have read and agree to the Eigent Contribution Guideline