Fix webhook 404s and changelog check crashes

[TylerZeroMaster]

Summary

staxly hadn't been redeployed to Heroku in ~2 years. This PR's first two commits (already on this branch) fixed the deploy-blocking issue; this commit fixes what that unmasked:

• WEBHOOK_PATH: Probot 13 changed its default webhook listen path from / to /api/github/webhooks. GitHub's webhook URL for this app is still configured for /, so every webhook POST 404'd after the upgrade. Added WEBHOOK_PATH=/ to app.json (both as a live fix and so future deploys/Review Apps don't regress).
• context.config() instead of probot-config: the abandoned probot-config dependency called context.github.repos.getContents(...), but context.github was renamed to context.octokit back in 2021. Replaced with Probot's built-in context.config() and dropped the dependency.
• context.pullRequest() instead of context.issue(): changedFiles was building pulls.listFiles params from context.issue() (issue_number), but that endpoint needs pull_number. Verified the fix produces the correct request URL.
• createCommitStatus instead of createStatus: repos.createStatus was renamed in current Octokit.
• robot.log.info(...) instead of robot.log(...): pino loggers aren't callable directly — this call has apparently never succeeded.

None of the last four bugs were introduced by this PR — they'd all been present since 2021, each one masking the next, invisible because changelog.js has never had any test coverage (/* istanbul ignore file */, no test file). This PR adds test/changelog.test.js to close that gap.

Heads up

staxly's own .github/config.yml requires a README.md update for any PR touching src/ (via this same changelog check) — a convention that hasn't actually been followed since 2023. So this PR will likely show changelog: pending on itself once the check is live again. That's expected and non-blocking (it's an informational status, not a required check) — decided not to address it in this PR; tracking it in a follow-up issue instead.

Test plan

• yarn jest --coverage — all suites pass
• standard --fix — clean
• Deploy and confirm webhooks land at / with no more 404s in Heroku logs
• Confirm a real pull_request event produces a changelog status instead of an error in the logs

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR restores correct operation of the Probot-based webhook app after a long gap in deployments by fixing webhook routing/config loading and updating the changelog status check to use current Probot/Octokit APIs, plus adding a new Jest test to cover the changelog check behavior.

Changes:

• Add WEBHOOK_PATH=/ to prevent webhook 404s after Probot’s default webhook path change.
• Replace probot-config usage with Probot’s built-in context.config() and update Octokit method usage (createCommitStatus, context.pullRequest()).
• Add a Jest test (test/changelog.test.js) to exercise the changelog status logic and prevent regressions.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
app.json	Adds WEBHOOK_PATH env var default to keep GitHub App webhook URL compatible.
package.json	Removes probot-config dependency now that context.config() is used.
yarn.lock	Drops transitive dependencies that were only required by probot-config.
src/changelog.js	Updates config loading, PR file listing, commit status API call, and logger invocation for current Probot/Octokit.
test/changelog.test.js	Adds coverage for changelog status behavior on pull_request events using nock.

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