Implement three-tier memory architecture (Hot/Vault/Handoff)

This commit replaces the previous memory_layers.py with a proper three-tier
memory system as specified by the user:

## Tier 1 — Hot Memory (MEMORY.md)
- Single flat file always loaded into system context
- Contains: current status, standing rules, agent roster, key decisions
- ~300 lines max, pruned monthly
- Managed by HotMemory class

## Tier 2 — Structured Vault (memory/)
- Directory with three namespaces:
  • self/ — identity.md, user_profile.md, methodology.md
  • notes/ — session logs, AARs, research
  • aar/ — post-task retrospectives
- Markdown format, Obsidian-compatible
- Append-only, date-stamped
- Managed by VaultMemory class

## Handoff Protocol
- last-session-handoff.md written at session end
- Contains: summary, key decisions, open items, next steps
- Auto-loaded at next session start
- Maintains continuity across resets

## Implementation

### New Files:
- src/timmy/memory_system.py — Core memory system
- MEMORY.md — Hot memory template
- memory/self/*.md — Identity, user profile, methodology

### Modified:
- src/timmy/agent.py — Integrated with memory system
  - create_timmy() injects memory context
  - TimmyWithMemory class with automatic fact extraction
- tests/test_agent.py — Updated for memory context

## Key Principles
- Hot memory = small and curated
- Vault = append-only, never delete
- Handoffs = continuity mechanism
- Flat files = human-readable, portable

## Usage

All 973 tests pass.

tests/test_agent.py

@@ -78,7 +78,8 @@ def test_create_timmy_embeds_system_prompt():
         create_timmy()
 
         kwargs = MockAgent.call_args.kwargs
-        assert kwargs["description"] == TIMMY_SYSTEM_PROMPT
+        # Prompt should contain base system prompt (may have memory context appended)
+        assert kwargs["description"].startswith(TIMMY_SYSTEM_PROMPT[:100])
 
 
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