feat(compile): allow callers to skip embeddings

[ethanj]

Takes over #169 by @TigerOfCountryYao, which had been a draft since 30 July with failing CI. His commit is preserved as the first commit here, rebased onto current main; the second commit is the test work.

What it does

compile({ embeddings: false }) runs page generation, links and the lexical index without any embedding-provider call or pending-embedding retry, for an SDK host that maintains its own semantic index. Omitting it is unchanged.

Why it is needed, which the original PR undersold

"Just don't set the embedding credential" does not work for two of the four backends:

Provider	Embedding credential	Can a caller opt out today?
anthropic / claude-agent	VOYAGE_API_KEY	Yes, don't set it
openai	OPENAI_EMBEDDINGS_API_KEY, then OPENAI_API_KEY	No - the chat key doubles as the embedding key
ollama	none required	No - embeddings always run

What changed from the original

It builds. test/compile-options.test.ts was missing its closing });, so esbuild could not transform it and build-and-test was red from the day the PR opened.

The default path is now pinned. The original had one case, asserting that embeddings: false does not refresh. That is one-directional: it passes against a build that never refreshes, so it cannot distinguish "the flag works" from "embeddings are broken for everyone". Mutation-tested to confirm — forcing the refresh to be skipped unconditionally left it green. Two cases now cover the default, and both mutants are caught: skipping unconditionally turns 2 red, ignoring the flag turns 1 red.

It uses FinalizeFlags. The original added a sixth positional boolean to finalizeWiki, adjacent to scoped. #192 turned those into a named object precisely so this one could join it as flags.embeddings rather than a same-typed neighbour that transposes silently.

embeddings is deliberately not a prompt modifier: it changes what runs after pages are written, never what a prompt asks for, so it must not enter the modifier digest or invalidate a page that is byte-identical under it.