Document bridge source tools

[HwangJohn]

Summary

• document llmwiki-agent-bridge read-only MCP source tools added in Expose read-only source tools llmwiki-agent-bridge#10
• explain sourceRegistry readiness metadata on /health and the A2A-style agent card
• add a quickstart smoke example for progressive source exploration without a runtime call
• add troubleshooting guidance for source IDs, registry readiness, and source-tool selection

Spec / ADR

• Spec: no new spec; docs-only follow-up for already-merged bridge source tooling.
• ADR: no new ADR; no ownership or protocol posture changes.

Contract impact

• Docs only. Describes additive bridge MCP tools and additive readiness metadata already merged in agent-bridge.

Validation

• npm run check
• git diff --check
• Sensitive-data review: changed files contain no private endpoints, local paths, or credentials.

LLMWiki / docs

• Public docs portal pages updated.
• No generated local LLMWiki artifacts included.