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).

apps/cli/tests/source-launch.compat.spec.ts

@@ -0,0 +1,36 @@
+import { fileURLToPath } from 'node:url'
+import { execa } from 'execa'
+import { describe, expect, it } from 'vitest'
+
+/**
+ * Keyless smoke for the SOURCE `dsh` launcher: run `apps/cli/src/bin.ts`
+ * with the exact production launch vector (`node --import tsx/esm`, the same
+ * shape as `bin/dsh` and the root `dsh`/`demo:tui`/`demo:web` scripts) and
+ * assert the piped-stdio TTY refusal. The Node compatibility matrix runs this
+ * WHOLE file, so a Node release changing module hooks or TypeScript handling
+ * breaks this gate instead of every developer's `pnpm dsh`; the built-bin
+ * suite covers the published `lib/` entry, not this source chain.
+ */
+
+const repoRoot = fileURLToPath(new URL('../../../', import.meta.url))
+const dshSourceBin = 'apps/cli/src/bin.ts'
+
+describe('dsh SOURCE launcher (node --import tsx/esm)', () => {
+  it('boots the source entry and refuses pipes LOUD (non-zero exit + stderr)', async () => {
+    const result = await execa(process.execPath, ['--import', 'tsx/esm', dshSourceBin], {
+      cwd: repoRoot,
+      input: '',
+      timeout: 25_000,
+      killSignal: 'SIGKILL',
+      reject: false,
+    })
+    if (result.timedOut) {
+      throw new Error(`dsh source launch did not exit within 25s. stdout:\n${result.stdout}\nstderr:\n${result.stderr}`)
+    }
+    expect(result.exitCode).not.toBe(0)
+    expect(result.stderr).toContain('requires stdin and stdout to be interactive TTYs')
+    expect(result.stderr).toContain('dsh -p')
+    // The refusal happens before any plugin mounts: stdout stays silent.
+    expect(result.stdout).toBe('')
+  }, 30_000)
+})