refactor(config): rename auto compact config key

Prefer the more user-friendly idleCompactAfterMinutes name for auto compact while keeping sessionTtlMinutes as a backward-compatible alias. Update tests and README to document the retained recent-context behavior and the new preferred key.

nanobot/config/schema.py

@@ -77,7 +77,12 @@ class AgentDefaults(Base):
     reasoning_effort: str | None = None  # low / medium / high / adaptive - enables LLM thinking mode
     timezone: str = "UTC"  # IANA timezone, e.g. "Asia/Shanghai", "America/New_York"
     unified_session: bool = False  # Share one session across all channels (single-user multi-device)
-    session_ttl_minutes: int = Field(default=0, ge=0)  # Auto /new after idle (0 = disabled)
+    session_ttl_minutes: int = Field(
+        default=0,
+        ge=0,
+        validation_alias=AliasChoices("idleCompactAfterMinutes", "sessionTtlMinutes"),
+        serialization_alias="idleCompactAfterMinutes",
+    )  # Auto-compact idle threshold in minutes (0 = disabled)
     dream: DreamConfig = Field(default_factory=DreamConfig)