security: add prompt injection guards, path traversal prevention, and input validation

Defense-in-depth security hardening for a codebase where markdown files become
LLM system prompts. Adds centralized security module, PreToolUse hook for
injection detection, and CI-ready codebase scan.

New files:
- security.cjs: path traversal prevention, prompt injection scanner/sanitizer,
  safe JSON parsing, field name validation, shell arg validation
- gsd-prompt-guard.js: PreToolUse hook scans .planning/ writes for injection
- security.test.cjs: 62 unit tests for all security functions
- prompt-injection-scan.test.cjs: CI scan of all agent/workflow/command files

Hardened code paths:
- readTextArgOrFile: path traversal guard (--prd, --text-file)
- cmdStateUpdate/Patch: field name validation prevents regex injection
- cmdCommit: sanitizeForPrompt strips invisible chars from commit messages
- gsd-tools --fields: safeJsonParse wraps unprotected JSON.parse
- cmdFrontmatterGet/Set: null byte rejection
- cmdVerifyPathExists: null byte rejection
- install.js: registers prompt guard hook, updates uninstaller

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/build-hooks.js

@@ -17,6 +17,7 @@ const DIST_DIR = path.join(HOOKS_DIR, 'dist');
 const HOOKS_TO_COPY = [
   'gsd-check-update.js',
   'gsd-context-monitor.js',
+  'gsd-prompt-guard.js',
   'gsd-statusline.js',
   'gsd-workflow-guard.js'
 ];