Add event, schedule and manual workflow triggers

Add event, schedule and manual workflow triggers

Workflows can now react to change requests, issues, comments, cron
schedules and manual runs, and a push trigger can filter by the paths it
changed. Branch and tag patterns gain ! exclusion.

Event runs are pinned with a JSON payload and the runner exposes it as
VOLE_EVENT, VOLE_EVENT_ACTION and VOLE_EVENT_PATH. Change-request runs
check out the base tip and carry the head and changed files, so an event
never executes an unapproved contributor commit.

Workflow dispatch resolves typed inputs and is available from the
workflows page and `vole workflow dispatch`. Deduplication now includes
an occurrence so the same commit can run for a later event or schedule
slot while a push remains idempotent.

Part of: #138