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/web/public/session-ui.js

@@ -92,7 +92,7 @@ const RUN_MODE_LAUNCH = {
   },
   deepseek: {
     label: 'DeepSeek',
-    installHint: 'DeepSeek Harness CLI (dsh) not found. Install with: npm install -g @deepseek-ai/dsh pnpm',
+    installHint: 'DeepSeek Harness CLI (dsh) not found. Install with: npm install -g @deepseek-ai/dsh && (command -v pnpm >/dev/null || npm install -g pnpm)',
     // The two-part availability check is deliberate. `dsh` being installed is
     // not enough — DeepSeek ships no terminal front door, so a box can have a
     // perfect binary and nothing a pane can run.