Add reversible content proposal mode

[StoneHub]

What changed

• adds an Add popup mode alongside Element, Visual, and Region
• lets a user anchor proposed text, image, list, or HTML/embed-frame placeholders to existing page content
• supports before, after, inside-start, and inside-end placement when valid
• records filler, support intent, acceptance checks, structured insert mutations, and before/proposed evidence
• restores the live page after Save, Cancel, navigation, or close
• keeps the HTML/embed frame non-executable and local-only
• updates History, AI Bundle formatting, packaging, docs, and the interactive UX contract

Why

Existing capture modes document or modify content already on the page. This adds the missing workflow for describing content that should exist and handing its placement and intent to a developer or coding agent.

Validation

• npm test
• npm run check
• npm run package
• npm run verify:package
• JavaScript syntax checks for changed runtime modules
• git diff --check

Per request, no unpacked-extension smoke test or new screenshots were included in this handoff. The PR should remain draft until the Add workflow receives an exact-browser runtime pass and is deliberately reconciled with the separate in-progress v1.8 Feedback Session work.

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