fix(provider-detection): detect CLIs on Windows via where (fixes #515)

[atharvwasthere]

Problem

On Windows, zeroshot providers reports every provider as ✗ not found, even when the CLI is installed and on PATH (claude --version works fine). This makes zeroshot unusable on Windows out of the box.

Fixes #515.

Root cause

lib/provider-detection.js probes for a CLI with the POSIX shell builtin command -v:

execSync(`command -v ${command}`, { stdio: 'pipe' });

On Windows execSync runs through cmd.exe, which has no command builtin, so the probe always throws and commandExists / getCommandPath always fail. The project already solves this correctly in src/preflight.js (process.platform === 'win32' ? 'where' : 'which'), but the provider-detection helpers never got the Windows branch.

Fix

Branch on platform, mirroring src/preflight.js:

const probe = process.platform === 'win32' ? `where ${command}` : `command -v ${command}`;

getCommandPath now takes the first line of output, since where can return multiple matches.

Tests

Adds tests/provider-detection.test.js (mocha + sinon) covering both probe branches and the multi-match path. All 6 pass; lint clean.

Verified locally on Windows 11 — after the fix, installed providers flip to ✓ found:

Claude    ✓ found   C:\Users\...\claude.exe
Codex     ✓ found   C:\Users\...\codex
Gemini    ✓ found   C:\Users\...\gemini
Opencode  ✓ found   C:\Users\...\opencode

Scope note (follow-up)

getHelpOutput / getVersionOutput in the same file call spawnSync(command, ...) without shell: true. On Windows that also fails for .cmd/.ps1 shims (most npm-installed CLIs). Left out to keep this PR focused on the reported detection bug; happy to add a follow-up if you'd like it bundled.