fix(spawn): pin Git Bash for Windows Terminal

[orangewk]

Summary

• resolve the Bash executable from the running Git Bash environment before launching Windows Terminal
• convert the MSYS/Cygwin path to a native Windows path with cygpath -w
• pass that explicit executable to both wt.exe and wt, avoiding Windows PATH resolution of bare bash
• add a regression test that simulates Git Bash and WSL Bash coexistence

Root cause

The Windows launcher used wt.exe new-tab bash -l .... Because wt.exe is native, bare bash is resolved through the Windows PATH and can select the WSL bash.exe shim. WSL Bash then cannot open the Git Bash/Windows-style boot-script path.

Impact

Windows users with Git Bash and WSL installed can use the default spawn.sh Windows Terminal path without a custom --terminal workaround. Custom terminal templates are unchanged.

Validation

• bats tests/test_spawn.bats --filter 'Windows Terminal receives' — pass
• bash -n scripts/spawn.sh — pass
• git diff --check — pass
• bats tests/test_spawn.bats — 73 pass, 2 pre-existing Windows-local failures
  • spawn: errors when $TMUX is set but tmux is not on PATH
  • spawn: refuses when the name is held by another live session
• both failures reproduce unchanged at the v1.1.13 merge commit (4bcfde9)
• local ShellCheck executable was unavailable; CI remains the static-analysis gate

Closes #39