ML-175: parse bash commands structurally via unbash

Replace regex-based command blocking with structural AST parsing
using unbash (zero-dependency TypeScript bash parser). The walker
collects command names only from executable positions (simple
commands, pipelines, subshells, command substitutions, control-flow
bodies), excluding arguments, comments, and quoted strings.

- Convert to directory-style extension with pinned unbash@3.0.0
- Add AST walker (ast.ts) covering 15 shell construct types
- Update blocked_commands to plain names instead of regexes
- Keep regex fallback for parse failures (degradation path)
- Path-based blocking unchanged for non-bash tools
- 41 tests cover blocked/allowed cases including false positives

.pi/extensions/sensitive-file-guard/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "sensitive-file-guard",
+  "private": true,
+  "type": "module",
+  "dependencies": {
+    "unbash": "3.0.0"
+  },
+  "devDependencies": {
+    "tsx": "^4.21.0"
+  }
+}