fix: CLI group settings handling and env var support (#257)

* Fix CLI group settings handling

* Document group settings in config

* fix: move group settings below channel list in README, add GROUP_DEBOUNCE_SEC env var

- README: group settings section was splitting the cross-channel bullet
  list; moved it after the channel table
- onboard.ts: groupDebounceSec had no env var override for non-interactive
  deploys; added <CHANNEL>_GROUP_DEBOUNCE_SEC for all 5 channels
- SKILL.md: documented the new env var

Written by Cameron and Letta Code

"For every complex problem there is an answer that is clear, simple, and wrong." - H.L. Mencken

---------

Co-authored-by: Jason Carreira <jason@visotrust.com>
Co-authored-by: Cameron <cameron@pfiffer.org>

README.md

@@ -211,6 +211,20 @@ Signal ────┘
 
 At least one channel is required. Telegram is the easiest to start with.
 
+### Group Settings (Optional)
+
+Configure group batching and listening mode in `lettabot.yaml`:
+
+```yaml
+channels:
+  slack:
+    groupDebounceSec: 5
+    instantGroups: ["C0123456789"]
+    listeningGroups: ["C0987654321"] # observe only, reply on mention
+```
+
+See `SKILL.md` for the full environment variable list and examples.
+
 ## Bot Commands
 
 | Command | Description |