Adds the three first-class PR workflow node kinds and their handlers via
createPrNodeHandlers(deps), registered in createDefaultNodeHandlers
(fail-closed pr-nodes-unwired when absent). GitHub ops are injected as
callbacks (PrNodeGithubOps) at all three CLI sites (daemon/serve/dashboard)
so the engine never imports the dashboard client (FN-3049). pr-create
routes open/failed as outcomes; pr-merge passes expectedHeadOid and never
writes 'merged' (reconcile corroborates); pr-respond delegates to an
injected respond callback (U5 fills the body). 10 tests.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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