Silence the canvas fallback prompt when Treemon reports a worktree unmonitored

[0101]

Problem

When Treemon is running but reports the current directory as unmonitored, the canvas extension still sends a browser-fallback URL to the session after every canvas write. If the host has its own canvas pane, that prompt pushes the agent to share a second, competing view of the same doc.

Changes

• Keep serving browser-fallback pages when Treemon reports the worktree unmonitored, but stop sending the URL to the session.
• Keep sending the URL when Treemon is unreachable, and say so in the prompt text.
• Add extension coverage for both browser-fallback branches.
• Update docs/spec/canvas-browser-fallback.md and the canvas skill to describe the two branches.

Tests

• npm run test:extension - 74 passed

[Copilot]

🟢 Approval recommended

The implementation, tests, and documentation consistently distinguish unreachable Treemon from explicitly unmonitored worktrees.

Pull request overview

This PR prevents competing canvas prompts when Treemon explicitly reports an unmonitored worktree while preserving browser fallback when Treemon is unreachable.

Changes:

• Adds registration-aware fallback prompt generation.
• Covers both fallback branches with tests.
• Updates the authoritative spec and canvas skill guidance.

File summaries

File	Description
src/Extension/skill/SKILL.md	Documents host-aware fallback behavior.
src/Extension/session-prompt.test.mjs	Tests silent and prompted fallback branches.
src/Extension/session-prompt.mjs	Generates prompts only when Treemon is unreachable.
src/Extension/extension.mjs	Applies registration state when handling canvas writes.
docs/spec/canvas-browser-fallback.md	Updates fallback behavior specification.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0
• Review effort level: Balanced

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