docs: refine README and research-git workflows

[sunjiajunsunjiajun]

Summary

• refine the README branding, motivation, quick start, and command guidance
• add realistic Codex examples for capture, recall, and recorded experiments
• document how experiment results stay linked to the code that produced them
• replace the workflow overview with compressed Capsule capture and recall diagrams

Validation

• git diff --check
• xmllint --noout for all added SVG assets

Scope

Documentation and visual assets only. No CLI, storage, or Capsule behavior changes.