fix: restore polyglot wrapper to fix Windows hook window spawning

Claude Code spawns hook commands with shell:true + windowsHide:true,
but on Windows the execution chain cmd.exe -> bash.exe causes Git
Bash (MSYS2) to allocate its own console window, bypassing the hide
flag. This creates visible terminal windows that steal focus on every
SessionStart event (startup, resume, clear, compact).

The fix:
- Rename session-start.sh to session-start (no extension) so Claude
  Code's .sh auto-detection regex doesn't fire and prepend "bash"
- Restore run-hook.cmd polyglot wrapper to control bash invocation
  on Windows (tries known Git Bash paths, then PATH, then exits
  silently if no bash found)
- On Unix, the polyglot's shell portion runs the script directly

This avoids Claude Code's broken .sh auto-prepend, gives us control
over how bash is invoked on Windows, and gracefully handles missing
bash instead of erroring.

Addresses: #440, #414, #354, #417, #293
Upstream: anthropics/claude-code#14828

hooks/hooks.json

@@ -6,7 +6,7 @@
         "hooks": [
           {
             "type": "command",
-            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh",
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/run-hook.cmd session-start",
             "async": false
           }
         ]