Upgrade probot from 12 to 14

[tykeal]

Problem

probot is at 12.2.8; latest is 14.3.2 (two majors behind). probot is
the core runtime dependency and pulls in much of the vulnerable
transitive tree flagged by npm audit (e.g. validator, ws).

Proposed change

Upgrade probot to ^14, address breaking API changes in index.js
and api/github/webhooks/index.js, and verify the app builds and the
Jest suite passes (100% coverage threshold). Validate a Vercel preview
deploy before merge.

Acceptance criteria

• probot on ^14; code updated for breaking changes.
• Tests pass at 100% coverage.
• npm audit high/critical count drops.
• Vercel preview deploy works.