Delete the mcode opt-in and dead helpers

[SaladDay]

Deletes a setting that gated nothing, plus helpers with no production caller.

• mcode execution opt-in. OAC_RUNTIME_MCODE_AGENTS_API was set to 1 by every producer: the native installer, the Runtime image and the integration test. Discovery already rejects every native version other than SupportedVersion. SupportsExecution is deleted, and an available mcode now declares its execution capabilities directly. The README, Dockerfile and installer environment drop the variable; the Dockerfile also drops OAC_RUNTIME_MCODE_WORKSPACE=managed, which nothing reads.
• Dead helpers. deadcode reported these unreachable, and a repo-wide grep (all modules) confirmed it:
  • deleted: installroot.Lock, cli environmentPathsOverlap, daemonize MustWriteLine, paths.SessionsFile, api.ParseInstallationConfiguration, v1.ValidateModelProtocol, sandbox.lowerHex, e2b.New, microsandbox.New, and the unused obs/log API (Info/Error/Debug/ChildSpan/Discard, ctxLogger.Debug/With, Carrier.ChildSpan, and the unused op argument of StartBackgroundTrace);
  • test-only uses: WritePIDFile/IsAlive, localworkspace.New and node.LoadIdentity tests now build the value directly, and runtimegateway.NewSession moves into session_test.go.
  • Kept because they have callers: providerassets.Catalog (Web), microsandbox.Labels (the microsandbox-provider module), and the coreErrorResponse marker. Codex and Claude SDK leftovers are cleaned up on the agent-outside-sandbox branch, which rewrites those adapters.
• dispatch. preparationState.workspaceReadOnly always equalled executor == nil, so the field is deleted.

Checks:

• gofmt; go build ./... (plus windows/darwin for ./apps/daemon/...); vet across the daemon, internal, Core, contracts and Web; build and vet the microsandbox-provider module.
• go test for the 15 touched packages, and -race for dispatch.
• make check-runtime-contract check-harness-catalog check-docs, check-names.py.
• deadcode now lists only the claudesdk prepare (owned by the feature branch) and the two kept items above.

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