Settle the agent-host placement review findings

[SaladDay]

Fixes from the blind review of #550, and the CI failure of #551.

• Idle compute (runtime_compute.go): the pass keeps the allocation and handles a wake request before it looks at the assignment, which it needs only to quiesce. An unplaced Session has nothing to quiesce, and a disconnected agent host is retried later; neither is recorded as provider_unavailable any more.
• Initialization placement (runtime_initialization.go): the scanner places a Session only on a host that advertises its Harness as available (runtimeDeclaration), the predicate the Turn path already uses. A host without the Harness no longer gets the Session pinned and failed with ProvisioningHarness; with no such host, the initialization waits like a queued Turn.
• Wake (runtime_compute_wake.go): waitServing waits at most 20 of the pass's 30 seconds, leaving time to resume the Environment, and returns the parent's error on shutdown or lease loss instead of ErrComputeUnconfirmed.
• Sync main bb1485ec into the cutover #551's CI failure: TestInitializationBindsAgentHost creates a hosted Session, which main's configured admission (Admit hosted work only on a configured deployment #546) admits only on a configured deployment; it now uses configuredStore.

Checks: go build ./...; go vet and unit tests of execution; integration tests for compute lifecycle, suspension, idle clock, initialization, placement, the local Environment worker, Link authority and agent host (the new unplaced-observation assertion fails without the idle fix); make check-names check-docs check-ci.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}