docs: add WordPress.org release readme

[chubes4]

Summary

• Add a WordPress.org-style readme.txt so Plugin Check no longer fails on missing plugin readme metadata.
• Add .distignore for release packaging so dev/test-only files stay out of the distributed plugin.
• Document the intentional Core-candidate wp_* / wp_agent_* public API naming that Plugin Check reports as prefix warnings.

Verification

• composer test
• homeboy lint --path /Users/chubes/Developer/agents-api@release-plugin-check-readme --extension wordpress --force-hot
• homeboy test --path /Users/chubes/Developer/agents-api@release-plugin-check-readme --extension wordpress --force-hot
• studio wp plugin check /Users/chubes/Developer/agents-api@release-plugin-check-readme --slug=agents-api --format=strict-json --exclude-directories=tests,.github,.claude,.datamachine,.git --exclude-files=.distignore,.gitignore,.git,AGENTS.md,CHANGELOG.md,composer.lock,phpstan.neon --skip-plugins=ai-provider-for-openai,ai-provider-for-opencode,data-machine,data-machine-code,frontend-agent-chat,intelligence,markdown-database-integration,mcp-adapter,static-site-importer,wp-ai-gateway

Plugin Check result: 0 errors, 58 warnings. Remaining warnings are expected prefix warnings for the intentional Core-candidate public API names and hooks.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafting the WordPress.org readme, release packaging exclusions, Core-candidate naming explanation, and running local verification commands. Chris reviewed the direction and requested the PR.