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/sensitive-paths.json

@@ -1,5 +1,5 @@
 {
-  "_comment": "Blocked paths for pi. Glob-like patterns, matched case-insensitively against absolute paths.",
+  "_comment": "Blocked paths for pi. Glob-like patterns, matched case-insensitively against absolute paths. blocked_commands are exact command names (not regexes), matched case-insensitively against structurally parsed bash AST command names.",
   "blocked_paths": [
     "mise.local.toml",
     "*.pem",
@@ -15,6 +15,6 @@
   ],
   "blocked_commands": [
     "printenv",
-    "\\benv\\b"
+    "env"
   ]
 }