feat(core): extract ShortcutsPlugin into @editorjs/shortcuts-plugin

[ilyamore88]

Closes #171.

Extracts the last remaining in-tree EditorjsPlugin out of packages/core into a standalone @editorjs/shortcuts-plugin package, mirroring the ClipboardPlugin extraction (#129).

What changed

• New package packages/plugins/shortcuts-plugin (@editorjs/shortcuts-plugin), scaffolded from clipboard-plugin (package.json, tsconfig{,.build,.eslint}.json, eslint.config.mjs, jest.config.ts, stryker.conf.mjs, .gitignore, README.md).
• Source moved verbatim — ShortcutsPlugin.ts → src/index.ts with zero content change (git detects a pure rename). The plugin already imported only from @editorjs/sdk, so no code edits were needed.
• New test suite src/index.spec.ts — 12 cases at 100% coverage (statements/branches/functions/lines), closing the pre-existing gap (there was no ShortcutsPlugin.spec.ts). Covers shortcut registration from tool options, keydown match/dispatch, first-match-wins, the IME-composition guard, IndexError tolerance vs. rethrow of other errors, and destroy().
• Core wiring — added the workspace:^ dependency, added project references in both tsconfig.json and tsconfig.build.json, switched the import to @editorjs/shortcuts-plugin, and removed the now-empty src/plugins/ directory. Registration order in Core is unchanged.
• CI — new .github/workflows/shortcuts-plugin.yml, same package-check.yml-based shape as clipboard-plugin.yml (include-mutations: true).
• OpenSpec — moved the "Keyboard shortcuts plugin" requirement out of specs/core/spec.md into a new specs/shortcuts-plugin/spec.md; openspec validate passes for all 12 specs.

Notes / scope

• Per the request, the plugin source is unchanged. The two reserved-but-unused private methods (#processBlockTool / #processBlockTune) are moved as-is; they carry no statements so coverage stays at 100%. Consequently the issue's suggestion to also test the reserved block-tool/tune shortcuts map isn't applicable yet — it's documented as reserved in the new spec instead.

Verification

• yarn workspace @editorjs/shortcuts-plugin lint:ci — clean
• yarn workspace @editorjs/shortcuts-plugin test:coverage — 12 passed, 100%
• yarn workspace @editorjs/shortcuts-plugin build — ok
• yarn workspace @editorjs/core lint:ci / build — clean
• yarn constraints — clean
• openspec validate — 12 passed

Follow-ups (not in this PR)

• matchKeyboardShortcut has no unit test in @editorjs/sdk.
• .github/scripts/should-run-ci.sh only scans depth-1 packages/*, so it's blind to dependency changes for packages/plugins/* and packages/tools/* (pre-existing).

🤖 Generated with Claude Code

[github-actions]

Unit Tests

Package	Coverage	Delta
@editorjs/shortcuts-plugin	100%	N/A
@editorjs/core	90.47%	0% ⚪️

Mutation Tests

Package	Mutation score	Dashboard URL
@editorjs/shortcuts-plugin	100.00% 🟢	Dashboard
@editorjs/core	0.00% 🔴	Dashboard