Make background.js Node-safe and add test for default blocks editor mode

[rawritude]

Motivation

• Allow background.js to be loaded in a Node test environment by avoiding runtime errors when Chrome APIs are absent.
• Ensure buildSystemPrompt behavior is covered when editorMode is omitted so the system defaults to Blocks guidance.
• Provide a simple test runner configuration so tests can be executed with node --test.

Description

• Add a hasChromeRuntime check and wrap Chrome-specific listeners so background.js is safe outside Chrome.
• Export buildSystemPrompt from background.js using module.exports for use in Node tests.
• Add a minimal package.json with a test script (node --test) and update tests/background.test.js to include a new test that verifies default Blocks guidance when editorMode is omitted.

Testing

• Ran npm test which invokes node --test.
• All automated tests passed.
• Test suite output: 4 tests passed and 0 failed.

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Codex Task