refactor(sse): route executor lookup through ExecutorRegistry (R0.3)

Adds open-sse/executors/registry.ts (Map-based registry mirroring
translator/registry.ts): the built-in table in executors/index.ts stays
declarative, every entry is registered at module load, and
getExecutor()/hasSpecializedExecutor() resolve through the registry.
DefaultExecutor fallback, its memoization, and the cloud-agent (#6699) /
search-provider (#10274) guards are unchanged.

Also fixes a latent lookup leak: the old object-literal lookup treated
Object.prototype names (constructor, toString, ...) as specialized
executors; the Map registry resolves them to the DefaultExecutor
fallback like any unknown provider.

Parity proof: executor-map golden (137 entries, byte-identical
before/after), check:known-symbols green, 1018 tests across the 65
executor test files green. Docs: OPEN_SSE_ARCHITECTURE factory section
corrected (it claimed generation from providerRegistry).

Refs #3501

docs/frameworks/OPEN_SSE_ARCHITECTURE.md

@@ -368,7 +368,7 @@ const result = await executor.execute({
 });
 ````
 
-The factory is generated from `config/providerRegistry.ts` which lists all 338 providers and their executor class.
+Resolution goes through the `ExecutorRegistry` (`executors/registry.ts`): every specialized executor is declared in the built-in table of `executors/index.ts` and registered via `registerExecutor(alias, instance)` at module load; `getExecutor()` consults the registry and falls back to a memoized `DefaultExecutor` for any provider without a specialized entry. The full alias → executor mapping is characterized by the golden test `tests/unit/executor-map-golden.test.ts`.
 
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