Apply the durable-interrupt-dispatch refactor: split the monolithic
webapp.py into a thin routing layer plus per-source handlers in
webhooks/{github,slack,linear}.py, and add completion.py, dispatch.py,
and reconcile.py. Reconcile fork divergence by keeping the Bedrock/
Fireworks cross-provider fallback, the no-agent-attribution prompt
policy, the dashboard-handoff re-export, and the Slack channel-info
cache. ci_autofix is restored on the new dispatch model in a later
commit.
Refs: #80
