Clarify agent documentation boundaries

[isty2e]

Summary

• narrow the README to product evaluation, installation, representative workflows, safety boundaries, and documentation routing
• make SKILL.md a standalone agent operating guide with task routing, canonical request examples, safety limits, recovery, and verification guidance
• add a protocol reference for CLI outputs, hashes, line anchors, exit behavior, and the structured error envelope
• keep advanced reference files optional so installations containing only SKILL.md still support ordinary workflows

Why

The README and agent skill previously duplicated large workflow and format-specific sections without clear ownership. Removing all duplication would make a standalone skill unusable, while retaining every detail in both surfaces would continue to create drift and unnecessary context cost.

This change keeps the execution-critical minimum in the skill, leaves product discovery in the README, and assigns exhaustive cross-cutting contracts to a lazily loaded reference.

Validation

• skill compatibility checks passed for Codex, Claude Code, OpenCode, and Antigravity
• all relative Markdown links resolve
• the standalone single-file edit request example reaches canonical target resolution
• cargo fmt --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo test --locked (2,101 tests passed)
• git diff --check