fix: add OLLAMA_HOST fallback and graceful degradation on missing model

[yvonnedevlinrh]

Summary

Fixes #61 - Dewey does not read the ecosystem-standard OLLAMA_HOST environment variable and hard-exits when the embedding model is missing. Users who configure Ollama via OLLAMA_HOST (which other tools like
uf doctor respect) found that Dewey always connects to localhost:11434. Additionally, when Ollama is reachable but the embedding model hasn't been pulled, dewey serve exits with a fatal error instead of degrading gracefully to keyword-only mode.

Changes

• OLLAMA_HOST fallback: Add ResolveOllamaEndpoint() helper to embed/config.go implementing the three-tier env var fallback chain: DEWEY_EMBEDDING_ENDPOINT > OLLAMA_HOST > default. Values without
  a URL scheme are normalized with http://.
• Default endpoint constant: Extract DefaultOllamaEndpoint from 5 hardcoded http://localhost:11434 strings across embed/, llm/, and cli.go.
• Graceful degradation: Convert the hard exit on missing embedding model to a warning log + keyword-only mode in both dewey serve (main.go:initObsidianBackend) and dewey index/reindex (cli.go:createIndexEmbedder). Warning includes model name, endpoint, and ollama pull instructions.
• Doctor consistency: Replace direct os.Getenv reads in dewey doctor with embed.ReadEmbeddingConfig() so doctor reports the same endpoint that serve/index resolve.
• Init template: Comment out endpoint in the dewey init scaffolded config.yaml so OLLAMA_HOST works out of the box for new users.

Testing

• go build ./... - PASS
• go vet ./... - PASS
• go test -race -count=1 ./... - PASS (all 17 packages)
• Gaze: CRAPload 41/48 (PASS), GazeCRAPload 45/45 (PASS)
• 10 new tests: 8 for endpoint resolution (including config.yaml vs OLLAMA_HOST precedence), 2 for graceful degradation (serve + index paths)
• Manual verification: OLLAMA_HOST resolves correctly, missing model degrades gracefully with warning

Change Artifacts

OpenSpec: openspec/changes/ollama-host-fallback/

##Fixes
#61

[gxmiranda]

Clean, well-scoped bug fix addressing both issues from #61: OLLAMA_HOST env var fallback and graceful degradation when the embedding model is missing.

Highlights:

• DefaultOllamaEndpoint constant eliminates 5 hardcoded endpoint strings
• Three-tier fallback chain (DEWEY_EMBEDDING_ENDPOINT > OLLAMA_HOST > default) with scheme normalization
• Graceful degradation to keyword-only mode in both serve and index paths
• Doctor now uses ReadEmbeddingConfig() for consistent endpoint reporting
• 10 new tests with mock Ollama servers covering precedence, normalization, and degradation paths
• Complete OpenSpec artifacts (proposal, design, spec, tasks)
• AGENTS.md updated with endpoint resolution precedence documentation

[LOW] Synthesis endpoint (llm/config.go) still uses DEWEY_EMBEDDING_ENDPOINT without OLLAMA_HOST fallback — explicitly deferred as out of scope per proposal. Should be tracked as a follow-up issue.

This review was generated by /review-pr (AI-assisted).