fix(llm-security): HIGH — bare root/home targets bypassed the rm block

The BLOCK rule named "Filesystem root destruction" did not block the bare
root target it is named for. The target alternation ended in a shared `\b`,
and a word boundary cannot hold after `/` or `~` at end-of-command.
Measured against the shipped pattern:

  bare `/` target        NOT blocked      `$HOME` target   blocked
  bare `~` target        NOT blocked      `/usr` target    blocked
  `/*` glob target       NOT blocked
  swapped flags (-fr)    NOT blocked
  sudo-prefixed          NOT blocked

Only targets whose first character is a word character ever satisfied the
assertion, so the rule caught `/etc` but not bare root. These fell through
to WARN (exit 0) — advisory only, command executed.

- Pattern: `\b` moved onto the `$HOME` alternative alone, where it is
  meaningful (it ends in a word char, so `$HOMEDIR` is still not
  swallowed). Dropped from `/` and `~`. Nothing else changes: `/etc`,
  `/home`, `./build` behave exactly as before.
- The old test file encoded this defect as expected behaviour, with a NOTE
  claiming the pattern "requires separate flag groups (e.g. -f -r, not -rf
  combined)". That diagnosis was wrong — the `/etc` case blocks fine with
  merged flags. Comment replaced with the real root cause.

npm test: 1872/1872 green (1865 + 7 new).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TcQyMTQfyrsAapaCMPxTtQ

hooks/scripts/pre-bash-destructive.mjs

@@ -21,7 +21,11 @@ import { getPolicyValue } from '../../scanners/lib/policy-loader.mjs';
 const BLOCK_RULES = [
   {
     name: 'Filesystem root destruction (rm -rf /)',
-    pattern: /\brm\s+(?:-[a-zA-Z]*f[a-zA-Z]*\s+|--force\s+)*-[a-zA-Z]*r[a-zA-Z]*\s+(?:\/|~|\$HOME)\b/,
+    // The target alternation must NOT end in a shared `\b`: a trailing word-boundary
+    // cannot hold after `/` or `~` at end-of-command, so the bare `rm -rf /` and
+    // `rm -rf ~` forms this rule is named for would fall through. `\b` belongs only on
+    // `$HOME`, which ends in a word character (so `$HOMEDIR` is not swallowed).
+    pattern: /\brm\s+(?:-[a-zA-Z]*f[a-zA-Z]*\s+|--force\s+)*-[a-zA-Z]*r[a-zA-Z]*\s+(?:\/|~|\$HOME\b)/,
     description:
       '`rm -rf /`, `rm -rf ~`, and `rm -rf $HOME` would destroy the entire filesystem ' +
       'or home directory. This command is unconditionally blocked.',