Add Step 5 flat context and txtai file tools for agents

docs/flat_file_context/STEP4_FLAT_FILE_CONTEXT_DESIGN.md

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+# Step 4 Flat File Context Design (Persona Data)
+
+## Intent
+Capture onboarding Step 4 persona outputs in an agent-first flat file so agents can quickly personalize strategy, content, and platform execution.
+
+## Storage location
+- `workspace/workspace_<safe_user_id>/agent_context/step4_persona_data.json`
+
+## Required Step 4 coverage
+- core persona profile (`core_persona`)
+- platform personas (`platform_personas`)
+- quality metrics (`quality_metrics`)
+- selected platforms (`selected_platforms`)
+- research persona/notes when available
+- source payload + timestamps for traceability
+
+## Agent summary expectations
+- quick facts: selected platform count, persona availability flags
+- retrieval hints: persona/profile adaptation queries
+- persona focus: compact actionable slice of core persona + quality constraints
+
+## Usage policy
+1. Start with `agent_summary`.
+2. Expand into `data` only when a task needs full fidelity.
+3. Use `document_context.related_documents` to fetch upstream Step 2/Step 3 context as needed.