Document agent execution flow and source entry points

[ChanTso]

The project overview now gives agent execution its own design entry and links the buyer and merchant loops, shared dispatch, protocol adapter, and budget implementation. The runtime guide explains complete-response validation before tool dispatch, result ownership, conversation and memory versions, and both native build entry points.

The English and Chinese overviews remain aligned. Historical and final acceptance records retain their separate versions and counts.

Validation:

• git diff --check passed.
• Checked 93 relative links and anchors, matching five-design sections, and unchanged Mermaid blocks.
• Two independent read-only reviews passed after correcting the conversation CAS description to match its version check.
• GitHub CI passed: Python, Web, Android, and Apple core/native build checks.

This change only edits Markdown. No application code, dependency, or measured result changed; no new local business tests or benchmarks were run.