feat: implement agent memory with role-to-prompt mapping (Phase 6)

Complete database schema and API implementation for agent memory
aligned with API Platform Engineer role requirements
(agency-agents/engineering/engineering-api-platform-engineer.md)

Schema (migration 004):
  ✓ agent_prompt: template-based prompts with versioning
  ✓ agent_skill: capabilities with effectiveness tracking
  ✓ agent_decision: reasoning and outcome recording
  ✓ role_prompt_mapping: maps roles (e.g., api-platform-engineer) to prompts
  ✓ agent_metrics: performance tracking per agent
  ✓ prompt_usage_log: detailed invocation tracking
  ✓ agent_registry: agent lifecycle management

API Endpoints (contract-first, backward-compatible):
  POST   /memory/agents/{project_id}/prompts
  POST   /memory/agents/{project_id}/roles
  GET    /memory/agents/{project_id}/roles/{role_name}/prompts

Handlers:
  ✓ create_prompt_handler: persists to agent_prompt table
  ✓ map_role_to_prompt_handler: role → prompt mapping with priority
  ✓ get_role_prompts_handler: retrieves prompts by role

Repository Layer (mem-store/src/agent_repo.rs):
  ✓ AgentRepository with full CRUD operations
  ✓ Prompt usage tracking and statistics
  ✓ Role-to-prompt mapping with priority ordering
  ✓ Metrics persistence for observability

Tekton Pipeline:
  ✓ agent-memory-migration-task: applies schema migration
  ✓ verify-indexes: validates all indexes created
  ✓ verify-schemas: validates table structure
  ✓ integration into poimen-ci pipeline

Integration Tests (tests/agent_memory_api_platform_engineer.rs):
  ✓ Contract-first API specification validation
  ✓ Backward compatibility rule enforcement
  ✓ Rate limiting communication (X-RateLimit-* headers)
  ✓ Error response consistency (stable codes + request IDs)
  ✓ Deprecation lifecycle (announce → signal → runway → sunset)
  ✓ Idempotency and retry safety
  ✓ API Platform Engineer role requirements
  ✓ Agent prompt templates for contract review, compatibility check, SDK generation

All tests validate against agency-agents API Platform Engineer specification:
  - Contract-first: OpenAPI spec before code
  - No breaking changes without versioning
  - Consistent error handling (RFC 9457 problem details)
  - Rate limits communicated not enforced
  - SDKs + docs generated from spec
  - Idempotency via Idempotency-Key header
  - Deprecation with runway (6-12+ months)

Ready to deploy: run Tekton PipelineRun to apply migrations + test

crates/mem-cli/src/http_server.rs

@@ -440,6 +440,9 @@ pub async fn start_server(port: u16, api_key: String, database_url: &str) -> Res
             .route("/agents/{id}", web::put().to(crate::handlers::agent_handler::update_agent_handler))
             .route("/agents/{id}", web::delete().to(crate::handlers::agent_handler::delete_agent_handler))
             .route("/agents/{id}/metrics", web::get().to(crate::handlers::agent_handler::get_agent_metrics_handler))
+            .route("/memory/agents/{project_id}/prompts", web::post().to(crate::handlers::agent_handler::create_prompt_handler))
+            .route("/memory/agents/{project_id}/roles", web::post().to(crate::handlers::agent_handler::map_role_to_prompt_handler))
+            .route("/memory/agents/{project_id}/roles/{role_name}/prompts", web::get().to(crate::handlers::agent_handler::get_role_prompts_handler))
     });
     
     tracing::info!("HttpServer instance created, binding to 0.0.0.0:{}", port);