docs: add embedding troubleshooting for stalled flows

[mason5052]

Summary

Adds a troubleshooting subsection to the Embedding Configuration and Testing section of the README for the case where a flow starts but then waits indefinitely with no progress. Documentation only.

Problem

In #322 a user reported that a flow "just keeps waiting, nothing moving ahead." As noted on the issue, this symptom is frequently caused by a misconfigured or unreachable embedding provider rather than by the flow itself, and the suggested first step is to check docker logs pentagi and the embedding configuration guide. The README documents how to configure and test embeddings, but it does not connect the "flow hangs / never progresses" symptom to embedding provider settings, so users debug the flow instead of the provider.

Solution

A new ### Troubleshooting: Flow Stalls or Hangs Without Progress subsection placed at the end of the Embedding Configuration and Testing section. It gives a short, ordered diagnostic path that matches the maintainer guidance on the issue:

1. Check docker logs pentagi for embedding errors (auth, wrong model, timeout, TLS).
2. Run etester test -verbose to validate the embedding provider and database connection without starting a flow.
3. Verify the relevant .env settings, with cross-links to the existing Supported Embedding Providers and Why Consistent Embedding Providers Matter subsections.

Every environment variable referenced (EMBEDDING_PROVIDER, EMBEDDING_MODEL, EMBEDDING_URL, EMBEDDING_KEY, OPEN_AI_KEY, OPEN_AI_SERVER_URL, PROXY_URL, HTTP_CLIENT_TIMEOUT) already exists in both .env.example and backend/pkg/config/config.go. The LLM-provider fallback note mirrors the behavior already documented just above in the same section.

User Impact

Documentation only. No new environment variables, no provider default changes, and no code or test changes. Users who hit a stalled flow get a direct, ordered path to the likely embedding-provider cause instead of debugging the flow.

Test Plan

• git diff --check reports no whitespace errors
• Diff is a single file (README.md, 29 insertions); no code, schema, compose, or .env.example changes
• Every referenced env var verified present in both .env.example and backend/pkg/config/config.go
• No new environment variables and no provider defaults changed
• Internal anchor links (#embedding-tester-utility-etester, #supported-embedding-providers, #why-consistent-embedding-providers-matter) resolve to existing headings

Refs #322

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Adds a README troubleshooting section to help diagnose flows that stall/hang due to embedding provider misconfiguration or connectivity issues.

Changes:

• Documented symptoms and likely root cause (embedding calls failing/hanging) for stalled flows
• Added step-by-step diagnostics: container logs + etester validation
• Added configuration checklist and guidance on reindex/flush when switching providers

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