feat(plugins): extension points, capability consent, async runtime and plugin API 1.0

Rework the plugin system per docs/plugin_plan.md (P0–P8):

- Extension-point registry (ide-extension) so new contribution types register
  without touching the kernel; languages, run detectors, providers, views and
  UI contributions go through it.
- Per-plugin capability consent with a risk-graded ledger; process
  capabilities are pinned to resolved binary paths and hashes.
- Async, actor-per-plugin Wasm runtime with lazy activation, cancellation,
  events, long-running tasks and idle reclamation.
- Plugin host moved into ide-host, shared by local and remote projects.
- Distribution: install/package tooling (openidea-plugin bin) and a static
  market index.
- WIT frozen as openidea:[email protected]; Rust SDK with mock host.
- Ten official validation plugins under plugins/official.

plugins/official/string-tools/.gitignore

@@ -0,0 +1 @@
+/target