Regen stubs workflow fails: buf not installed, no decree checkout

[zeevdr]

Description

The Regen stubs workflow (.github/workflows/regen-stubs.yml) has failed on every decree-released dispatch (2026-06-19, 2026-06-21, 2026-06-30). It fails at npm run generate with sh: 1: buf: not found (exit 127) — buf is not installed in the job. A second gap sits behind it: the workflow never checks out opendecree/decree, where the protos live, so buf generate would have no proto input even once buf is installed.

This is pre-existing and non-urgent: the decree v0.12.0-alpha.5 release changed no protos, so the committed stubs are not stale. The break only matters on the next actual proto change. A companion fix is tracked for the Python SDK.

Acceptance criteria

• buf is installed in the job (for example, via bufbuild/buf-setup-action).
• The workflow checks out opendecree/decree so buf generate has its proto input.
• A workflow_dispatch trigger with a version input is added so the workflow can be tested without cutting a release.
• A manual workflow_dispatch run completes green: stubs regenerate, npm test passes, and a pull request is opened (a no-op pull request is acceptable when protos are unchanged).

[zeevdr]

Companion fix for the Python SDK: opendecree/decree-python#167 (same root cause — regen-stubs.yml never checks out the decree repo).