Add passive mesh context awareness — observe channel traffic, inject into LLM prompts

New context.py module: ring buffer (50K hard cap, ~25MB ceiling) passively
records all channel broadcasts. Observations are formatted with relative
timestamps and injected into the system prompt when generating LLM responses.
Only public channel traffic is observed; DMs to the bot are excluded (already
in per-user history). Bot's own node ID is auto-added to ignore list.

Config: context.enabled, observe_channels, ignore_nodes, max_age, max_context_items
TUI: new Context settings submenu (menu item 7)
Hourly prune removes expired observations.

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

docker-entrypoint.sh

@@ -43,6 +43,13 @@ memory:
   window_size: 4
   summarize_threshold: 8
 
+context:
+  enabled: true
+  observe_channels: []
+  ignore_nodes: []
+  max_age: 2592000
+  max_context_items: 20
+
 llm:
   backend: openai
   api_key: ""