[Finomosec]

Closes #10165.

npx <pkg> / npm exec -- <pkg> now replaces the npm process with the command instead of spawning it and waiting:

before                          after
npm exec <pkg>                  <pkg>
└─ sh -c '<pkg>'
   └─ <pkg>

• lib/commands/exec.js asks for this only for a single command; --workspaces keeps running one child per workspace.
• libnpmexec passes it on to @npmcli/run-script, except in interactive mode (no command).
  For a plain bin call with the default sh it also prefixes the script with exec, so the shell is replaced as well; --call scripts and custom script-shells are left as they are.
• On Windows, and until the dependencies below are released, everything behaves as before.

Besides the memory of the idle npm process, this also fixes orphaned processes: today npm only forwards SIGINT and SIGTERM, and the shell in between does not forward anything, so a SIGKILL (or a SIGTERM that reaches only the shell) leaves the actual program running with PPID 1.
MCP clients run into this a lot, because MCP servers are typically configured as npx -y <pkg> (see anthropics/claude-code#76306 and anthropics/claude-code#1935).
With this change the program is the process the caller started, so signals reach it directly.

Things that no longer happen after the handover, because npm is gone: the update notice and npm's own "command failed" messages on a non-zero exit.
The exit code is the program's own.

Depends on npm/promise-spawn#175 and npm/run-script#283; they need to be released and bundled for this to take effect.
The workaround for nodejs/node#66824 lives in promise-spawn.

Tested:

• test/lib/commands/exec.js and libnpmexec at 100 % coverage on Node.js 22, 24 and 26.
• End-to-end with the bundled dependencies patched, on Node.js 22, 24 and 26: no npm or shell process left in the tree, blocking stdio, large output into a slow pipe, exit code and argument quoting unchanged, SIGTERM/SIGKILL leave no orphans, first-time install prompt/warning still shown, --call, interactive mode and --workspaces unchanged.