Add database migrations for session context system

Phase 1: Database Schema Implementation
- Migration 010: Sessions table with bulletproof auto-incrementing
  - Unique session IDs across all projects
  - Per-project session numbers (1, 2, 3, ...)
  - Atomic sequence generation (no race conditions)
  - Session-task and session-commit linking
  - Semantic search with pgvector HNSW indexes

- Migration 011: Memories table with enhanced schema
  - Migrated existing session_memories to memories
  - Added session_id and task_id foreign keys
  - Renamed columns for consistency
  - HNSW indexing for semantic search

- Migration 012: Builds table for CI/CD tracking
  - Links builds to sessions and versions
  - Tracks build status, timing, and metadata

All migrations tested and verified on agiliton database.

Related: CF-167 (Fix shared session-summary.md file conflict)

migrations/011_memories.sql

@@ -0,0 +1,58 @@
+-- Migration 011: Memories/Learnings table with semantic search
+-- Purpose: Migrate existing session_memories to memories table with enhanced schema
+-- Dependencies: 001_base_schema.sql (pgvector), 010_sessions.sql (sessions table)
+
+-- Rename existing session_memories table to memories
+ALTER TABLE IF EXISTS session_memories RENAME TO memories;
+
+-- Add missing columns
+ALTER TABLE memories
+    ADD COLUMN IF NOT EXISTS task_id TEXT REFERENCES tasks(id) ON DELETE SET NULL;
+
+-- Rename source_session to session_id for consistency
+ALTER TABLE memories
+    RENAME COLUMN source_session TO session_id;
+
+-- Add foreign key constraint to sessions table
+ALTER TABLE memories
+    ADD CONSTRAINT fk_memories_session
+    FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE SET NULL;
+
+-- Rename times_surfaced to access_count for consistency
+ALTER TABLE memories
+    RENAME COLUMN times_surfaced TO access_count;
+
+-- Rename last_surfaced to last_accessed_at for consistency
+ALTER TABLE memories
+    RENAME COLUMN last_surfaced TO last_accessed_at;
+
+-- Change varchar columns to TEXT for consistency
+ALTER TABLE memories
+    ALTER COLUMN project TYPE TEXT;
+
+-- Note: Keeping embedding as vector(1536) to preserve existing data
+-- Future embeddings can use vector(1024) by updating the column if needed
+
+-- Add missing indexes
+CREATE INDEX IF NOT EXISTS idx_memories_session ON memories(session_id);
+CREATE INDEX IF NOT EXISTS idx_memories_task ON memories(task_id);
+CREATE INDEX IF NOT EXISTS idx_memories_project ON memories(project);
+CREATE INDEX IF NOT EXISTS idx_memories_category ON memories(category);
+CREATE INDEX IF NOT EXISTS idx_memories_created ON memories(created_at DESC);
+
+-- Drop old index and create HNSW index for better performance
+-- Note: Existing index is ivfflat, we want hnsw
+DROP INDEX IF EXISTS idx_session_memories_embedding;
+CREATE INDEX IF NOT EXISTS idx_memories_embedding ON memories USING hnsw (embedding vector_cosine_ops);
+
+-- Full-text search on title and content (for keyword search)
+CREATE INDEX IF NOT EXISTS idx_memories_fts ON memories USING gin(
+    to_tsvector('english', title || ' ' || content)
+);
+
+-- Comments for documentation
+COMMENT ON TABLE memories IS 'Learnings and patterns discovered during development sessions';
+COMMENT ON COLUMN memories.category IS 'Type of memory: pattern (reusable solution), fix (bug resolution), preference (user choice), gotcha (trap/pitfall), architecture (design decision)';
+COMMENT ON COLUMN memories.embedding IS 'Vector embedding of title + content for semantic search';
+COMMENT ON COLUMN memories.context IS 'Optional context describing when/where this memory applies';
+COMMENT ON COLUMN memories.access_count IS 'Number of times this memory has been retrieved (for relevance ranking)';