Closes #12

Problem

Daemon processes don't source shell profiles (.zshrc/.bashrc), so exec.LookPath fails for binaries installed via version managers like nvm, mise, etc. This causes DetectAndConfigure to miss claude even when it's installed — on a fresh/empty config, the agent is never detected.

Root Cause

exec.LookPath only searches the current process's PATH. When weclaw runs as a daemon (Setsid: true), the PATH doesn't include nvm's bin directory (e.g. ~/.nvm/versions/node/v24.11.1/bin/), because those paths are added in .zshrc which is only sourced for interactive shells.

Solution

Add a lookPath() helper that falls back to resolving via a login interactive shell (zsh -lic which <binary>) when the standard exec.LookPath fails. The -li flags ensure .zshrc/.bashrc are sourced, picking up version manager paths.

Test Plan

Added config/detect_test.go with 4 test cases:

• TestLookPath_InPath — finds standard binaries already in PATH
• TestLookPath_NotExist — returns error for missing binaries
• TestLookPath_LoginShellFallback — reproduces daemon scenario: strips PATH to system-only dirs, verifies exec.LookPath fails but lookPath succeeds via login shell
• TestDetectAndConfigure_StrippedPath — end-to-end: empty config + stripped PATH → claude detected as CLI agent