fix(cli): scope PR update to current generator and fix CI concurrency

[davidkonigsberg]

Description

Refs: Devin session
Requested by: @davidkonigsberg

Fixes the test-remote-vs-local-generation-parity workflow that has been failing since Feb 9 (when PR #11502 / v3.70.0 introduced the PR update feature for self-hosted GitHub generation).

Root cause: When multiple generators (ts-sdk, java-sdk, go-sdk, python-sdk) run concurrently in CI, they all target fern-api/empty and findExistingUpdatablePR finds the same most-recently-updated PR for all of them. They all try to push to the same branch, causing non-fast-forward rejections on all but the first push. Retries don't help because each retry hits the same race.

Changes Made

• Generator-scoped branch prefixes: Branch naming changes from fern-bot/{date} to fern-bot/{generator-name}/{date} (e.g. fern-bot/fernapi-fern-typescript-sdk/2026-02-13_...). findExistingUpdatablePR now filters by this generator-specific prefix, so generators only find and update their own PRs.
• Workflow concurrency: Changed cancel-in-progress: true → false. With GitHub Actions, this lets the current run finish while cancelling any intermediate pending runs, so only the newest queued run executes next (instead of constantly cancelling in-progress jobs).
• CLI versions.yml: Added v3.77.1 changelog entry.

Review Checklist

• Backward compatibility: Existing PRs with old fern-bot/{date} branches won't be matched by the new prefix filter. First generation after this change will create a new PR instead of updating existing ones — acceptable one-time transition?
• generatorName is optional: Falls back to "fern-bot/" (old behavior) when undefined. In practice, generatorInvocation.name is always defined at the call site — verify this is safe.
• Branch name sanitization: Only replaces / with -. Are there other characters in generator names that could be invalid in git branch names?
• Same-generator race: This fix prevents cross-generator races but doesn't address two concurrent runs of the same generator. Not an issue for the test workflow (one job per generator), but worth considering for production.

Testing

• Lint passes (pnpm run check)
• CI workflow validation (requires full environment with tokens and fern-api/empty repo — will validate post-merge)

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