docs: clarify agent guide contract and workflow routes

[SWBaek]

Summary

Agents editing CLI contracts could miss the public manual, schemas, examples, and tests; document-schema edits lacked a validator regeneration route; and the implementation issue Status rule left project selection implicit. AGENTS.md now names those routes and selects the repository-linked sdoc-editor project.

Closes #242.

Validation

• Validated with: npm run verify:all on Windows with Node 22.22.2 and npm 10.9.7. Includes verify:fast (the npm run check target), npm run build:all, UI, Extension Host, VSIX, and CLI packaging checks.
• 883 unit tests, 72 Chromium UI tests, and 14 VS Code Extension Host scenarios passed.
• Validated with: npm run repo:check, git diff --check, and command/path/anchor review.
• All five PR CI jobs passed for cfa8039; the tested tracked files match that commit.
• Tests for changed document, persistence, or converter behavior: not applicable to this guide-only patch.

The isolated Windows checkout needed npm run validators:generate to undo checkout CRLF conversion before the freshness check. This produced no Git content change. Details are recorded in #242.

Contribution rights

• I have the right to submit this work under the repository's MIT license.
• The change contains no credentials, personal data, or unapproved proprietary assets.