Add Phase 7: polish and hardening — retry, truncation, sessions, shutdown

- Config extensions: retry backoff, truncation threshold, session persistence
- LLM retry with exponential backoff + jitter on transient errors (5xx, connection)
- Conversation truncation: drops oldest messages preserving first user + recent N
- Session persistence: auto-save/restore with atomic writes, cleanup of old files
- Graceful shutdown: SIGTERM handler, cancel() on AgentLoop, save-on-exit
- Partial message recovery on mid-stream interruption
- New slash commands: /save, /session
- 18 new tests (5 retry, 5 truncation, 4 session, 4 integration workflows)
- README.md and docs/tools.md documentation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config/config.yaml

@@ -7,11 +7,16 @@ llm:
   temperature: 0.1
   max_tokens: 4096
   timeout: 120
+  max_retries: 3
+  retry_backoff_base: 1.0
+  retry_backoff_max: 30.0
 
 agent:
   max_iterations: 25
   max_conversation_tokens: 32000
   workspace_root: "."
+  truncation_keep_recent: 10
+  truncation_threshold: 0.85
 
 permissions:
   auto_approve:
@@ -56,6 +61,12 @@ tools:
     max_file_size_bytes: 1048576  # 1 MB
     binary_detection: true
 
+session:
+  session_dir: ".sneakycode/sessions"
+  auto_save: true
+  max_session_age_hours: 72
+  offer_resume: true
+
 display:
   show_tool_calls: true
   show_token_usage: true