chore: move VSCode extension to canopy-dashboard repo

[ashmitb95]

Summary

The VSCode extension lives at ashmitb95/canopy-dashboard now. This PR completes the canopy-side cleanup:

• Deletes vscode-extension/ (62 files, ~22.6k lines)
• Drops the extension-build job from .github/workflows/ci.yml (the extension has its own CI on the new repo)
• README: adds an inline pointer to the new repo next to the Marketplace link

What moved + what was preserved

• git filter-repo --subdirectory-filter vscode-extension preserved all 26 commits of extension-only history in canopy-dashboard
• The parked vscode-extension/package.json description rewrite carried over as a regular commit in the new repo
• README + repository URL + image URLs updated in the new repo for standalone life
• New repo gained:
  • Its own CI (type-check + jest + esbuild + vsce package) at .github/workflows/ci.yml
  • A pipx-based Canopy: Install CLI command that opens an integrated terminal and runs pipx install canopy-cli && canopy setup-agent. Wired into the viewsWelcome and the MCP-failure toast as the primary install path. The managed-venv installer stays as a fallback.
  • cliResolver disk-scan fix: looks for ~/.local/pipx/venvs/canopy-cli/bin/canopy (the actual pipx venv name) in addition to the legacy ~/.local/pipx/venvs/canopy/bin/canopy

Why split

• The extension and the CLI/MCP are at different release cadences and toolchains. Extension is at 0.7.1, CLI is at 3.1.1. Different publish surfaces (Marketplace vs PyPI), different languages.
• Schema coupling stays manageable: both sides agree on .canopy/state/*.json shapes. We discussed schema_version as the durable contract — that's a follow-up.

Test plan

• canopy-dashboard CI is green (build + jest + esbuild + vsce package)
• Local python -m build of canopy is unaffected (no Python files touched)
• After merge: no stale references to vscode-extension/ anywhere in canopy