chore(release): bump to v7.3.0

Batch C release. Closes 12 implementation tasks (E3, E8-E14, 8.4, 8.6,
8.7, 8.10) across four execution waves: A (bash + decoder), B (supply
chain + workflow scanner), C (MCP cumulative drift), D (code quality).

Wave E (9 new attack-simulator scenarios for the new defenses) deferred
to v7.3.1 — defenses are unit-tested per wave; the deferred work adds
attack-simulator regression coverage on top, not the primary safety net.

Tests: 1665+ → 1777 (Wave A-D cumulative, +112).

Version sync targets touched:
- package.json
- .claude-plugin/plugin.json
- CLAUDE.md (header)
- README.md (badge + new release-history row)
- scanners/ide-extension-scanner.mjs (VERSION constant)
- ../../README.md (marketplace root plugin entry)
- CHANGELOG.md (new [7.3.0] section per Keep a Changelog, all 12 task
  IDs covered individually under Added/Changed/Documentation/Tests/Notes)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/llm-security/.claude-plugin/plugin.json

@@ -1,5 +1,5 @@
 {
   "name": "llm-security",
   "description": "Security scanning, auditing, and threat modeling for Claude Code projects. Detects secrets, validates MCP servers, assesses security posture, and generates threat models aligned with OWASP LLM Top 10.",
-  "version": "7.2.0"
+  "version": "7.3.0"
 }