fix(cli-install): address review: drop every npm_config_prefix spelling, probe async

- Delete every spelling of npm_config_prefix before setting NPM_CONFIG_PREFIX, in the Compose
  branch too: npm run exports the lowercase key and a sorting /bin/sh let it win. The
  operator guard stays on the uppercase key only.
- A prefix that does not exist yet is judged by its nearest existing ancestor.
- The probe is async (promisified execFile, fs.promises.access, SIGKILL on timeout), awaited
  before the spawn and only for commands that run npm.
- Tests: lowercase/mixed-case keys, and the decision logic against a fake npm on PATH
  (writable, read-only, not yet created, npm missing). Docs: one clause in cli-registry.md.

docs/cli-registry.md

@@ -25,7 +25,7 @@ Every run mode Codeman can launch — Claude Code, Terminal/Shell, OpenCode, Cod
 App Settings → Agents & CLIs → **CLI management** (`cliManagementEnabled`, default OFF; admin-only in multi-user mode) lists every entry with an installed/not-installed badge and:
 
 - toggles any entry on or off. A `kind: 'shell'` entry cannot be disabled, and the row shows no switch for it. A disabled CLI disappears from the Run menu, the welcome screen and the phone overview, and new session requests for it are rejected.
-- installs a missing **stock** CLI by running its shipped install command, after a confirm that names the exact command. Only one install per CLI runs at a time, and the command runs without any `CODEMAN_*` variable in its environment. A custom entry's install command is never executed.
+- installs a missing **stock** CLI by running its shipped install command, after a confirm that names the exact command. Only one install per CLI runs at a time, and the command runs without any `CODEMAN_*` variable in its environment. An `npm install -g` command is pointed at `~/.local` (which every resolver searches) when the npm global prefix is not writable by the server user, for example a system node under `/usr`; a writable prefix, a prefix that does not exist yet but could be created, and an explicit `NPM_CONFIG_PREFIX` are left alone. A custom entry's install command is never executed.
 - adds, edits and deletes **custom** entries (id, label, badge, binaries, launch argv). The server re-validates the whole assembled entry through `CliEntrySchema`, so the form cannot bypass the load-time rules.
 
 These are the only writes to `clis.json`. They are serialized, and a file that does not parse or has unsafe permissions is refused rather than overwritten; fix it (or `chmod 600` it) and retry. The HTTP routes are listed in `docs/api-reference.md` under *CLI management*.