Fix 429 (model pin) and bot ignoring encrypted admin replies

Two independent bugs:

1. 429 "long context" — agentic sessions read enough files to exceed the
   200K standard window; on the default [1m] model Synapse escalates into
   the paid 1M tier. Pin claude_model to claude-opus-4-8 (200K window)
   so Claude Code compacts context instead. Updated config.example.json
   and the maintainer.py default / comment accordingly.

2. Bot silently ignores operator replies — the DM room was created with
   is_direct:true, causing Element to auto-enable E2E encryption. This
   stdlib-only bot cannot decrypt m.room.encrypted events, so all admin
   messages were dropped without any log output. Fix:
   - register-matrix-bot.sh now creates rooms with is_direct:false and
     preset:private_chat (Synapse does not force encryption on non-DM
     rooms).
   - matrix_poll_admin() now logs a loud WARN if an encrypted event
     arrives, instead of silently skipping it.

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

config.example.json

@@ -4,7 +4,7 @@
   "agent_file": "agents/stack-support.agent.md",
   "runbook_file": "docs/stack-support-runbook.md",
   "claude_bin": "/home/paul/.local/bin/claude",
-  "claude_model": null,
+  "claude_model": "claude-opus-4-8",
   "claude_extra_args": [],
   "agent_timeout_seconds": 1800,