Add per-project Brain viewer in Automations (#DIS-160)

[selfcontained]

Summary

• Adds Brains as the 3rd tab in the Automations section — a full project-level brain viewer with collection-first organization
• Sidebar lists all projects with brain data, showing object/list/event count badges
• Project detail view includes collection picker pills, search filter, and collapsible Objects/Lists/Events sections
• Agent attribution links on all brain entries navigate back to the agent session
• "View full brain →" link added to the per-agent Brain sidebar tab (from DIS-159)
• Live updates via existing brain.changed SSE event invalidation

Routing

/automations/brains → overview
/automations/brains/:encodedRepoRoot → project brain (all collections)
/automations/brains/:encodedRepoRoot/:collection → project brain, specific collection

Test plan

• Type check passes (pnpm run check)
• Production build passes (pnpm run finalize:web)
• E2E tests pass (113/114 — 1 pre-existing flaky failure in agent-routing)
• Unit tests pass
• Playwright validation: tab switching, project selection, collection filtering, search, list expansion, agent links

Closes DIS-160

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