fix: ensure standard PATH is available for external CLIs

[jicaiji1-max]

Problem

When calling external CLIs (like opencode), child processes may not find system commands like sysctl because the PATH environment variable does not include standard system paths.

This causes errors like:

error: Executable not found in $PATH: "sysctl"

Solution

Add standard paths to the environment when spawning external CLI processes.

Test Case

# Register opencode
opencli register opencode --binary /usr/local/bin/opencode --desc "OpenCode AI coding agent"

# Before fix: fails with sysctl not found
opencli opencode --version

# After fix: works correctly
opencli opencode --version
# Output: 1.2.20

Fixes #284

[Astro-Han]

Good fix for a real problem — sysctl not found makes sense when PATH is stripped. Two things to watch for:

process.env.PATH can be undefined

If PATH is unset, the template string produces "undefined:/usr/local/bin:..." — a literal undefined directory in the search path. A quick guard:

const currentPath = process.env.PATH || '';

isBinaryInstalled() uses a different PATH

The new env is only passed to spawnSync, but isBinaryInstalled() (line 68, uses which) still runs with the original process.env. If a binary exists only in a standard path like /usr/sbin, isBinaryInstalled returns false while spawnSync would find it — leading to unnecessary install prompts.