ARM Cortex M* microcontroller development (#79)

* expert at Teensy microcontroller programming

* Refactor teensy-expert to arm-cortex-expert for broader ARM Cortex-M coverage

- Rename teensy-expert.md to arm-cortex-expert.md
- Update frontmatter: remove triggers field, add model: sonnet
- Generalize content for ARM Cortex-M platforms (Teensy, STM32, nRF52, SAMD)
- Add critical ARM Cortex-M sections:
  * Interrupt Priorities & NVIC Configuration
  * Critical Sections & Interrupt Masking (with BASEPRI formula)
  * Hardfault Debugging Basics
  * Cortex-M Architecture Differences comparison table
  * FPU Context Saving
  * Stack Overflow Protection
- Optimize content: 471 lines → 264 lines (26% reduction)
- Preserve all critical code patterns for correct LLM code generation
- Inline all technical content (no external snippet dependencies)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

* Add arm-cortex-expert to README and marketplace

- Added arm-cortex-expert to Specialized Domains section in README
- Created arm-cortex-microcontrollers plugin entry in marketplace.json
- Plugin authored by Ryan Snodgrass for ARM Cortex-M firmware development

---------

Co-authored-by: Claude <noreply@anthropic.com>

.claude-plugin/marketplace.json

@@ -1814,6 +1814,34 @@
       "agents": [
         "./agents/elixir-pro.md"
       ]
+    },
+    {
+      "name": "arm-cortex-microcontrollers",
+      "source": "./agents",
+      "description": "ARM Cortex-M firmware development for Teensy, STM32, nRF52, and SAMD with peripheral drivers and memory safety patterns",
+      "version": "1.2.0",
+      "author": {
+        "name": "Ryan Snodgrass",
+        "url": "https://github.com/rsnodgrass"
+      },
+      "homepage": "https://github.com/wshobson/agents",
+      "repository": "https://github.com/wshobson/agents",
+      "license": "MIT",
+      "keywords": [
+        "embedded",
+        "arm",
+        "cortex-m",
+        "firmware",
+        "microcontroller",
+        "teensy",
+        "stm32"
+      ],
+      "category": "languages",
+      "strict": false,
+      "commands": [],
+      "agents": [
+        "./arm-cortex-expert.md"
+      ]
     }
   ]
 }