Step 5: Memory — SQLite persistence, embeddings, hybrid retrieval, summarization

## Context

Steps 1-4 are complete (types, config, providers, tools, runtime). The agent can think and act but forgets everything when it stops. Step 5 adds persistent memory.

## Scope

### SQLite schema + migrations
- Sessions table (id, name, created_at, updated_at, archived)
- Conversation events table (session_id, role, content, tool_calls, created_at)
- Memory chunks table (id, content, source, tags, embedding blob, created_at)
- FTS5 virtual table on chunks
- Session state table (summary, summary_epoch, scratchpad)
- Schema version tracking

### Embedding backends
- `EmbeddingBackend` interface implementation
- OpenAI embeddings (`text-embedding-3-small`)
- Ollama embeddings (local, free — `nomic-embed-text`)
- Configured via `memory.embedding_backend` in config

### Hybrid retrieval pipeline
- Vector search via sqlite-vec (cosine similarity)
- FTS5 search (BM25 scoring)
- Normalize scores (min-max within each result set)
- Weighted merge (configurable `vector_weight` + `fts_weight`)
- Return top_k results

### Rolling summarization
- Detect when context budget is approaching limit (chars/4 estimate already in runtime)
- Summarize unsummarized messages since last epoch
- Call provider with summarization prompt
- Update session state with new summary + epoch
- Replace old messages with summary in context

### Memory tools
- `memory_search` (ReadOnly) — search persistent memory
- `memory_save` (SideEffecting) — save information to memory

### Conversation persistence
- Store conversation events as they happen
- Load conversation history when resuming a session
- Session create/get/list/update/archive via `SessionStore` interface

### Scratchpad
- Per-session key-value scratchpad for agent working notes
- Persisted in session_state table

## Interfaces already defined
- `internal/types/memory.go` — `Memory`, `MemoryRetrieval`, `EmbeddingBackend`
- `internal/types/session.go` — `SessionStore`, `SessionRecord`

## Files to create
```
internal/memory/
├── sqlite.go # SQLiteMemory implementing MemoryRetrieval
├── schema.go # DDL + migrations
├── embedding_openai.go
├── embedding_ollama.go
├── hybrid.go # Hybrid retrieval pipeline
├── summarizer.go # Rolling summarization
└── memory_test.go

internal/tool/
├── memory_search.go # memory_search tool
└── memory_save.go # memory_save tool
```

## Dependencies
- `modernc.org/sqlite` (pure Go SQLite)
- `github.com/nicholasgasior/govec` or similar for sqlite-vec

## Not included
- Multi-user isolation
- Audit logging
- Cost tracking per memory operation