feat(pi): add the Pi agent runtime with native workflow and retrieval engines

Introduce an isolated Node worker that hosts the Pi SDK while Go keeps
business state, permissions, MCP execution, evidence and delivery checks.
The worker is published per source fingerprint, streams events back to Go
and supports tool callbacks, cancellation, session resume and per-model
transport settings.

Agent dispatch, tool search, plan/task tools and migration controls are
wired on top of it: large tool catalogs load through Pi tool_search, the
conversation task board keeps its existing tools, and legacy mode/route
values still resolve. Workflow gains a native DAG with versioned JSON
checkpoints, and knowledge retrieval moves to native documents, splitter,
HTTP embeddings, vector search and reranking.

.github/workflows/pi-runtime.yml

@@ -0,0 +1,27 @@
+name: Pi runtime
+on:
+  push:
+    paths: ['runtime/pi/**', 'internal/agentruntime/**', 'internal/multiagent/**', 'internal/knowledge/**', 'internal/workflow/**', 'internal/config/**', 'cmd/build-runtime/**', 'scripts/build-pi-runtime.sh', '.github/workflows/pi-runtime.yml']
+  pull_request:
+    paths: ['runtime/pi/**', 'internal/agentruntime/**', 'internal/multiagent/**', 'internal/knowledge/**', 'internal/workflow/**', 'internal/config/**', 'cmd/build-runtime/**', 'scripts/build-pi-runtime.sh', '.github/workflows/pi-runtime.yml']
+permissions:
+  contents: read
+jobs:
+  runtime:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22.19.0'
+          cache: npm
+          cache-dependency-path: runtime/pi/package-lock.json
+      - uses: actions/setup-go@v5
+        with:
+          go-version-file: go.mod
+      - run: npm ci --ignore-scripts
+        working-directory: runtime/pi
+      - run: npm run build && npm test
+        working-directory: runtime/pi
+      - run: go run ./cmd/build-runtime
+      - run: go test -race ./internal/agentruntime ./internal/multiagent ./internal/knowledge ./internal/workflow