perf(skills): use embed_batch in embed_skills_with_timeout

[bug-ops]

Summary

• embed_skills_with_timeout (crates/zeph-skills/src/embedding.rs) now tries LlmProvider::embed_batch first, wrapped in a single timeout for the whole batch, instead of embedding each skill sequentially — providers with a native batch endpoint (e.g. OpenAI) now cost one round trip instead of N.
• Falls back to the original per-item sequential loop (extracted unchanged into embed_skills_sequential) on batch error, timeout, or a mismatched result count, preserving the existing partial-failure tolerance (one bad or slow skill no longer loses the rest).
• Both callers (SkillMiner, trace-based skill extraction) are unaffected — the public function signature is unchanged.

Adversarial critique confirmed the result set is byte-identical to the original in every traced case; two minor, non-blocking notes worth surfacing:

• Worst-case latency on the timeout→fallback path is now (N+1)×timeout (fresh budget for the fallback) instead of N×timeout.
• A flaky provider could be double-charged on the failure-path re-embedding (batch attempt + fallback retry).

Note: found a pre-existing, unrelated rustdoc broken-intra-doc-link issue in crates/zeph-skills/src/matcher.rs ([Self::Qdrant] referencing a variant absent under some feature combinations), confirmed to exist on unmodified main via git stash. Out of scope here; filing as a separate follow-up issue.

Closes #6481

Test plan

• cargo nextest run -p zeph-skills — 573/573 passed (5 new tests, including a regression test proving partial-failure tolerance survives the batch→fallback transition)
• cargo +nightly fmt --check clean
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings clean
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins — 14805/14805 passed
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler" clean
• gitleaks protect --staged --no-banner --redact — no leaks