Document the three quality-push flags in config.toml.example

[jstuart0]

Summary

config.toml.example documents the two Phase-3 agentic flags (AGENTIC_RETRIEVAL_ENABLED, AGENTIC_RETRIEVAL_CANARY_PCT) but is missing the three quality-push flags that shipped in v0.9.0-rc.1:

• SOURCEBRIDGE_QA_PROMPT_CACHING_ENABLED
• SOURCEBRIDGE_QA_SMART_CLASSIFIER_ENABLED
• SOURCEBRIDGE_QA_QUERY_DECOMPOSITION_ENABLED

Operators copying config.toml.example to config.toml have no way to discover these knobs without reading the Go source.

What needs to happen

1. In config.toml.example, add a commented block for each flag in the [qa] section, following the style of the existing AGENTIC_RETRIEVAL docs (description, default, how to toggle).
2. Include a short note on what each flag does and the trade-off:
   • Prompt caching: pure cost win, safe default on for Anthropic, no quality risk.
   • Smart classifier: adds a small Haiku call per ask; improves seed context routing. See docs/admin/server-side-qa-rollout.md.
   • Query decomposition: architecture-class questions split into sub-questions and run in parallel; controlled by isDecomposableKind in internal/qa/agent_glue.go.
3. Add a pointer to docs/admin/server-side-qa-rollout.md for the full recommended prod posture.

Where to look

• config.toml.example — bottom of the [qa] section, right after the AGENTIC_RETRIEVAL docs (around line 135).
• internal/config/config.go — authoritative docstrings for each flag (QAConfig.PromptCachingEnabled, etc.).
• docs/admin/server-side-qa-rollout.md — the "Quality-push surgical config" section already has the recommended defaults; link to it.

Acceptance

• Each of the three flags has a multi-line comment block in config.toml.example matching the style of existing flag docs.
• A fresh contributor reading config.toml.example can understand what each flag does and when to toggle it without opening Go files.

Difficulty

Beginner-friendly. Pure documentation — no code changes. Good first PR to learn the repo's docs conventions.