feat(keys): default keymap for pccx commands

[hkimw]

Refs #128.

Summary:

• Add default shortcuts for the four read-only v002.1 PCCX commands.
• Lock the command-to-key mapping in the VS Code prototype manifest test.

Validation:

• node -e "JSON.parse(require('fs').readFileSync('editors/vscode-prototype/package.json', 'utf8')); console.log('package json ok')"
• for test_file in editors/vscode-prototype/test/*.test.mjs; do node "$test_file"; done
• npx --yes @vscode/vsce@3.2.2 ls --no-dependencies