Support generic caller sandbox invocation

[chubes4]

Summary

• Adds a generic browser_runner.invocation contract for browser Playground sessions so callers can invoke a sandbox-local Ability or caller-owned hook task.
• Keeps runtime.mu_plugins as the extension seam and proves caller MU plugin installation happens before invocation.
• Chooses a safe ephemeral Playground port range to avoid adjacent worker-port collisions when multiple runtimes overlap.

Verification

• npm run build
• npm run wordpress-plugin-smoke (240 assertions)
• npm run runtime-snapshot-restore-smoke
• npm run artifact-contract-smoke
• npm run runtime-action-adapter-smoke
• npm run recipe-dry-run-smoke
• Attempted npm run check; the aggregate command exceeded the 15-minute tool timeout after passing build, plugin smoke, browser runtime, package, artifact verifier, artifact patch, and while inside artifact-contract smoke. The timed-out smoke was rerun independently and passed.

Closes #317.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the generic invocation implementation, smoke coverage, documentation updates, and verification commands; Chris remains responsible for review and merge.