ci: opt-in Blender 5.1 smoke via needs-5.1 and dispatch

[TMHSDigital]

Summary

Closes #134. Opt-in 5.1 smoke without putting 5.1 on every PR.

This repo has no ci.yml. The existing lever is blender-smoke.yml's empty workflow_dispatch: {}. This PR adds a series choice input there, plus a needs-5.1 label path. pull_request.types was previously unset (GitHub default: opened / synchronize / reopened only). labeled is now listed explicitly so applying the label starts a run. Monday cron is unchanged. Default PR matrix stays 4.5 and 5.2.

Auto-label does not apply needs-5.1. It still only applies path labels (ci, documentation, ...). GITHUB_TOKEN label events do not retrigger workflows; a human (or REST) applying needs-5.1 does.

B2 gate-failure proof (live-run-proven)

needs-5.1 is left applied on this PR so the final head keeps a 5.1 job.

1. Absent by default (no needs-5.1)

gh pr checks 137 immediately after open, before the label. No Blender 5.1 smoke row. Smoke run: https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306483 (jobs: Resolve smoke matrix, Blender 4.5, Blender 5.2 only).

Blender 4.5 smoke	pending	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306483/job/103546936589
Blender 5.2 smoke	pending	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306483/job/103546936593
Auto-label by path	pass	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306490/job/103546923313
Ecosystem drift check	pass	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306577/job/103546923553
Resolve smoke matrix	pass	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306483/job/103546923482
Socket Security: Project Report	pass
Socket Security: Pull Request Alerts	pass
Validate content counts	pass
Validate plugin manifest	pass
Validate smoke harness protocol	pass
Validate structure and frontmatter	pass

That same run later finished 4.5 pass 3m10s / 5.2 pass 4m27s. Still no 5.1 job.

2. Triggers on label

Applied needs-5.1 via REST (gh pr edit --add-label hit the projectCards GraphQL deprecation). Labeled run matrix log: Smoke matrix: ["5.1"]. 5.1 job appeared and completed.

Labeled run: https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691361129

Blender 5.1 smoke	pass	4m17s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691361129/job/103547080593
Resolve smoke matrix	pass	3s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691361129/job/103547070877
Blender 4.5 smoke	pass	3m10s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306483/job/103546936589
Blender 5.2 smoke	pass	4m27s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691306483/job/103546936593

3. Canary turns 5.1 red, then revert is green

Canary: SystemExit(1) at the start of examples/bmesh-gear main() (0d32dbd). Synchronize with the label on, matrix all three series.

Failing run: https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691589680

Blender 4.5 smoke	fail	1m15s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691589680/job/103547687638
Blender 5.1 smoke	fail	1m14s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691589680/job/103547687642
Blender 5.2 smoke	fail	1m5s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691589680/job/103547687660
Resolve smoke matrix	pass	3s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691589680/job/103547675650

Revert dc8072c. Green run: https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691713403

Blender 4.5 smoke	pass	3m16s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691713403/job/103548023249
Blender 5.1 smoke	pass	4m21s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691713403/job/103548023220
Blender 5.2 smoke	pass	3m57s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691713403/job/103548023215
Resolve smoke matrix	pass	4s	https://github.com/TMHSDigital/Blender-Developer-Tools/actions/runs/34691713403/job/103548010665

Documentation (inspection)

One how-to section in CONTRIBUTING.md after Blender Version Targeting (that is where contributors already look for version policy). Pointers: workflow file header, README 5.1 table row + examples blurb, AGENTS.md CI/CD bullet, PR template evidence line.

Evidence labels

• live-run-proven: B2 check lists and run URLs above; 5.1 is not vacuous
• inspection-only: no ci.yml in this repo; auto-label allow-list; Monday cron cron expression unchanged; default matrix still 4.5+5.2 on unlabeled PRs

Out of scope

Agent bridge. LICENSE / #131. Baking/UV (Phase 5). VERSION / CHANGELOG / release.yml. Monday cron schedule and default PR version matrix. Examples/skills/snippets/rules/templates except the reverted canary. Exit-code cleanup is #136, not this PR.