feat(hub): drop mode/sdkLoader/SdkDispatchFailed (#175) (#220)

* feat(hub): drop mode/sdkLoader/SdkDispatchFailed (#175)

The Command Routing Hub no longer carries dual-runtime selection.
Removes `mode` and `sdkLoader` constructor parameters and the
`SdkDispatchFailed` and `SdkLoadFailed` errorKind values. The Hub
now routes exclusively through the CJS registry / handler resolution
path. ERROR_KINDS enum shrinks from 6 to 4 values.

phase-command-router.cjs updated to construct the Hub without the
removed params (removes tryLoadSdk, getExecuteForCjs, sdkLoader fn,
mode variable, and the post-dispatch SDK output branch). The 7
remaining family routers (init, phases, roadmap, state, validate,
verify, cjs-command-router-adapter) do not use createHub directly
and require no changes.

The CJS↔SDK bridge (bin/lib/cjs-sdk-bridge.cjs) is unchanged and
remains separately invokable; its removal is tracked in Phase 4 (#190).

ADR-0012 is no longer amended in this PR — the decision is captured
in ADR-0174 (which supersedes ADR-0012 entirely as part of the
SDK-retirement migration). Amending a superseded ADR would be
redundant noise.

Tests:
- Added assertions that Hub rejects/ignores `mode` and `sdkLoader`
- Removed obsolete mode-selection branching tests
- 57/57 local tests pass

Closes #175.
Part of #174 (ADR-0174).

* chore(changeset): add docs-exempt marker (#175)

P1.1's CommandRoutingHub work has no docs/ touchpoints — the
architectural decision is captured in ADR-0174 (merged via PR #198).
Per-phase ADR amendments would create noise; the SDK-retirement
migration's docs land in Phase 6 PRs (#193-#196) once the relevant
state is removed.

Adds the standard <!-- docs-exempt: <reason> --> marker inside the
changeset fragment so lint:docs accepts the PR without forcing a
docs/ touch that would be redundant.

Refs #175. Part of #174 (ADR-0174).

.changeset/lucky-birds-bark.md

@@ -0,0 +1,8 @@
+---
+type: Removed
+pr: 175
+---
+**`CommandRoutingHub` no longer carries dual-runtime selection** — the `mode`, `sdkLoader`, and `SdkDispatchFailed` errorKind are removed. The Hub routes exclusively through CJS handlers. No change to the observable `dispatch()` contract.
+
+<!-- docs-exempt: Hub is an internal seam; the architectural decision is documented comprehensively in ADR-0174 (merged via PR #198 — superseded ADR-0005/0007/0012/3524 as one canonical record for the SDK retirement). Per-phase ADR amendments would create noise; phase-specific docs land in Phase 6 PRs (#193-#196). -->
+