Add Step 5 flat context and txtai file tools for agents

docs/flat_file_context/STEP5_FLAT_FILE_CONTEXT_DESIGN.md

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+# Step 5 Flat File Context Design (Integrations)
+
+## Intent
+Capture onboarding Step 5 integration configuration in a compact agent-readable context so agents can reason about connected services and execution constraints.
+
+## Storage location
+- `workspace/workspace_<safe_user_id>/agent_context/step5_integrations.json`
+
+## Required Step 5 coverage
+- integration map (`integrations`)
+- provider list (`providers`)
+- connected account references (`connected_accounts`)
+- integration status and notes
+- source payload and timestamps
+
+## Agent summary expectations
+- connected integration count/list
+- provider count
+- retrieval hints for integration readiness checks
+
+## Linked traversal
+Use `document_context.related_documents` and `context_manifest.json` to navigate Step 2/3/4 upstream dependencies when deciding tool execution paths.