docs: explain streaming timeout and keep-alive behavior

[HisenWeb]

Summary

Documents the streaming timeout behavior introduced by PR #11.

This is documentation-only. No runtime behavior changes.

Changes

• Clarifies CODEX_OPENCODE_TIMEOUT_SECONDS semantics for non-stream and stream requests.
• Documents the 15-second downstream SSE keep-alive.
• Documents the generated Codex stream_idle_timeout_ms = 360000 setting.
• Calls out that existing ~/.codex/config.toml files may still contain the old 120000 value and must be regenerated or updated manually.
• Adds a dedicated streaming timeout and troubleshooting guide.
• Updates the documentation index and diagnostics checklist.

Files

• README.md
• docs/INDEX.md
• docs/DIAGNOSTICS.md
• docs/STREAMING_TIMEOUTS.md

Validation

• Compared against main after PR fix(streaming): prevent SSE idle disconnects and total stream timeouts #11 merge.
• Documentation values match the merged runtime defaults and generated configuration.
• No source, test, dependency, workflow, or install-script behavior is changed.