Add editor post slug target resolution

[chubes4]

Summary

• Add runtime-resolved post-slug=<slug> editor targets for wordpress.editor-open and editor validation callers.
• Resolve slugs with the requested post-type inside the running WordPress sandbox, then open the concrete post editor URL.
• Cover parsing, successful resolution, and no-match failure in editor action tests.

Tests

• npm run test:editor-actions

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode
• Used for: Drafted and verified the runtime target resolver change and tests under Chris's direction.