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>

app/models/config.py

@@ -16,6 +16,9 @@ class LLMConfig(BaseModel):
     temperature: float = Field(default=0.1, description="Sampling temperature")
     max_tokens: int = Field(default=4096, description="Maximum tokens in LLM response")
     timeout: int = Field(default=120, description="Request timeout in seconds")
+    max_retries: int = Field(default=3, description="Max retry attempts on transient errors")
+    retry_backoff_base: float = Field(default=1.0, description="Base seconds for exponential backoff")
+    retry_backoff_max: float = Field(default=30.0, description="Maximum backoff seconds")
 
 
 class AgentConfig(BaseModel):
@@ -28,6 +31,12 @@ class AgentConfig(BaseModel):
     workspace_root: Path = Field(
         default=Path("."), description="Root directory for file operations"
     )
+    truncation_keep_recent: int = Field(
+        default=10, description="Number of recent messages to preserve during truncation"
+    )
+    truncation_threshold: float = Field(
+        default=0.85, description="Token budget fraction that triggers truncation"
+    )
 
 
 class PermissionsConfig(BaseModel):
@@ -60,6 +69,19 @@ class ToolsConfig(BaseModel):
     filesystem: FilesystemToolConfig = Field(default_factory=FilesystemToolConfig)
 
 
+class SessionConfig(BaseModel):
+    """Session persistence configuration."""
+
+    session_dir: Path = Field(
+        default=Path(".sneakycode/sessions"), description="Directory for session files"
+    )
+    auto_save: bool = Field(default=True, description="Auto-save session after each turn")
+    max_session_age_hours: int = Field(
+        default=72, description="Max age in hours before session files are cleaned up"
+    )
+    offer_resume: bool = Field(default=True, description="Offer to resume previous sessions on startup")
+
+
 class DisplayConfig(BaseModel):
     """Terminal display preferences."""
 
@@ -76,6 +98,7 @@ class AppConfig(BaseModel):
     permissions: PermissionsConfig = Field(default_factory=PermissionsConfig)
     tools: ToolsConfig = Field(default_factory=ToolsConfig)
     display: DisplayConfig = Field(default_factory=DisplayConfig)
+    session: SessionConfig = Field(default_factory=SessionConfig)
 
     @model_validator(mode="after")
     def resolve_workspace_root(self) -> "AppConfig":