ci: dispatch PR validation manually; never queue on PR events

[ScriptedAlchemy]

Why the prior two cuts still failed

GitHub creates/queues a workflow run before evaluating job-level conditions. #1744 made drafts skip all jobs; the run objects still queued. #1773 made ci-full the only triggering label; the automated PR swarm applied ci-full to every PR. At 10:00: 173 open PRs, 80 queued CI runs, beta.45 publish blocked. I cleared 81 runs manually; publish immediately started and shipped all 9 assets.

Only robust GitHub-native policy

No PR event trigger exists for expensive validation:

• ci.yml: default-branch push + workflow_dispatch.
• plugin-validation.yml, sdk-conformance.yml: path-filtered default-branch push + dispatch.
• all three hotpath workflows: dispatch only.
• only pr-run-cleanup (closed) and read-only release-PR integrity retain PR events.

Run a PR head explicitly:

gh workflow run ci.yml --ref <branch>
# optional additions:
gh workflow run ci.yml --ref <branch> -f run_os=true -f run_hosts=true -f run_perf=true

Opening, pushing, marking ready, and labels now create no run. A CI dispatch runs repository gates, harness self-tests, and Linux. Default-branch pushes run Linux only, so release matrix jobs retain runner slots. OS/host/perf are explicit.

More deletion

• Removed dead pr-dogfood: without PR context it could never run.
• debug-cli runs only with run_hosts=true, its only remaining consumers.
• MCP conformance moved to stock-host lane on that shared binary.
• Removed dead PR commit-range step (local hook and default-branch push range check remain).
• Hotpath profile compares dispatched head with origin/master and records branch metadata rather than empty PR fields.

Net workflow diff: 65 additions, 272 deletions.

actionlint on all workflows, release safety, Rust cache-lineage guard pass. CONTRIBUTING.md updated.

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