feat(cli): launch dsh source through the tsx ESM hook

Node 26.0.0 removed --experimental-transform-types, so the native
source-launch chain cannot start anywhere on that line, and strip-only
mode rejects the vendored syntax (parameter properties, decorators,
runtime enums/namespaces). Switch bin/dsh, the root dsh/demo:tui/
demo:web scripts, and the Code Mode TUI overlay to node --import
tsx/esm: one launch vector across the whole engines range, ~0.4s faster
than the full tsx default (the CJS hook stays off; the graph is
ESM-only).

Delete scripts/tspath-loader.ts and apps/cli/src/tsconfig-paths-loader.ts:
tsx owns both transformation and tsconfig paths projection. Add
dsh-source-launch-smoke to the node-compat gates so the 22.19/26 matrix
executes the real launch vector; no CI job did, which is how the Node 26
breakage shipped silently.

Supersedes the native-TypeScript-source-launch Agent Note (new note
records the profiling evidence and rejected alternatives).

scripts/run-gates.ts

@@ -287,6 +287,11 @@ function nodeCompatSmokeGates(): Gate[] {
       'run',
       'packages/session-persistence/session-persistence-jsonl/tests/zstd.compat.spec.ts',
     ], { label: 'JSONL Zstandard smoke' }),
+    pnpmExec('dsh-source-launch-smoke', [
+      'vitest',
+      'run',
+      'apps/cli/tests/source-launch.compat.spec.ts',
+    ], { label: 'dsh source-launch smoke' }),
   ]
 }