fix(#616): install pnpm only when none is on PATH

The DeepSeek install command asked npm for dsh and pnpm in one go. On a
machine that already has pnpm through corepack (`corepack enable` puts a
pnpm shim in node's own bin directory, which is npm's global bin for nvm
and Homebrew installs), `npm install -g pnpm` refuses to overwrite that
shim with EEXIST, and because both packages were in the same command the
whole install failed: dsh was not installed either. Measured with npm 10
against a scratch prefix holding a corepack-style shim. On master the same
machine installed dsh fine and the profile install used the existing pnpm.

The command is now `npm install -g @deepseek-ai/dsh && (command -v pnpm
>/dev/null || npm install -g pnpm)`, so pnpm is installed only when the
install environment has none, and a failed dsh install still fails the
command. The two hints that embed it follow (the launch-table drift test
requires that), and the catalogue is regenerated; install.sh withholds
the command for a launcher-only CLI, so its block is unchanged.

The test now runs the real stock string through /bin/sh with a fake npm
on a PATH it controls, for the three cases: no pnpm, an existing pnpm,
and a failing dsh install.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/config/cli-registry/stock.ts

@@ -1261,8 +1261,12 @@ const DEEPSEEK: CliEntry = {
       command: {
         // pnpm rides along: `dsh plugin` spawns a literal `pnpm` with no npm fallback, so the
         // terminal profile this CLI needs before it can run cannot be installed without it (#352).
-        linux: 'npm install -g @deepseek-ai/dsh pnpm',
-        darwin: 'npm install -g @deepseek-ai/dsh pnpm',
+        // ⚠️ Only when none is on PATH. `corepack enable` puts a `pnpm` shim in node's own bin
+        // directory, which is npm's global bin for nvm and Homebrew, and `npm install -g pnpm`
+        // refuses to overwrite it (EEXIST). Asked for in the same command as dsh, that failure
+        // took dsh down with it, on exactly the machines that already had a working pnpm.
+        linux: 'npm install -g @deepseek-ai/dsh && (command -v pnpm >/dev/null || npm install -g pnpm)',
+        darwin: 'npm install -g @deepseek-ai/dsh && (command -v pnpm >/dev/null || npm install -g pnpm)',
       },
       npmPackage: '@deepseek-ai/dsh',
       docsUrl: 'https://github.com/deepseek-ai/deepseek-harness',