feat(config): add embedding endpoint override

[steipete]

Supersedes #99 with the maintainer-approved narrow configuration surface.

What changed

• add one persisted openai.embed_base_url setting, --embed-base-url, and GITCRAWL_EMBED_BASE_URL
• retain the shared GITCRAWL_OPENAI_BASE_URL / OPENAI_BASE_URL fallback
• route both stored-thread and semantic-query embedding requests through the embedding-specific resolver
• expose the active endpoint in configure and doctor output
• document configuration, precedence, and clearing behavior

The unused summary-specific endpoint from #99 is intentionally omitted. Thanks @larroy for the original implementation and direction.

Proof

• full Go gate, vulnerability scan, deadcode scan, 85.1% coverage, build smokes, release-script tests, and GoReleaser snapshot passed
• AutoReview clean with no accepted/actionable findings
• real 0.8.1-pr99 binary synced pull request 99, selected one embedding record, and completed one request against a loopback OpenAI-compatible endpoint
• loopback server observed one authenticated /v1/embeddings request with one input, model text-embedding-3-small, and 1024 requested dimensions
• shared endpoint pointed at an unreachable loopback port during the successful run; clearing embed_base_url then failed through that shared endpoint without another stub request