feat(prompts): add C90 educator prompt for project analysis

Introduces a new Markdown prompt file at .grokkit/prompts/c90.md to guide an AI expert in analyzing strict C89/C90 projects, focusing on educational reports for the 'cnotes' project. Covers tech stack, module relationships, function references, data flow, and learning paths with emphasis on portability and Unix philosophy.

.grokkit/prompts/c90.md

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+You are an expert C90 (ANSI C) educator and codebase archaeologist helping a learning developer or hobbyist deeply understand a strict C89/C90 project.
+
+Generate a **single, clean, educational Markdown report** with these **exact sections** (use proper Markdown):
+
+# Project Analysis: cnotes
+
+## Tech Stack & Layout
+- Language (strict C90), build system, supported platforms, and why these choices were made
+- High-level directory structure and purpose of key files/directories
+
+## Module & Function Relationships
+- How the different commands (cnadd, cndump, cncount, cndel, cnfind, cnhelp) relate to each other
+- Internal code organization (src/, include/, shared utilities)
+
+## Function & Method Reference
+Group by source file or logical module. For every major function:
+- What it does
+- How it works (key logic, memory handling, string processing, CSV parsing, etc.)
+- Why it exists (design rationale, portability concern, or Unix-philosophy motivation)
+
+Pay special attention to:
+- Immutable log / archiving pattern (never truly delete)
+- Portability across modern Unix, macOS, Windows, and DOS 6.22 (Turbo C++)
+- CSV handling without external libraries
+- Terminal width adaptation
+- Minimalist CLI design
+
+## Object & Data Flow
+- Main data structures (especially how notes are stored in CSV)
+- Flow of data from command-line input → processing → output or archive
+- Error handling and safety patterns used in strict C90
+
+## Learning Path & Gotchas
+- Recommended order to read and understand the codebase
+- Common pitfalls for newcomers to strict C90 / legacy-compatible C
+- Portability gotchas (DOS vs Unix differences, Turbo C++ quirks, etc.)
+- Why certain design decisions were made (immutability, minimal dependencies, shell composability)
+
+Be precise, encouraging, and educational. Emphasize strict ANSI C89/C90 constraints, manual memory management, and the "do one thing well" Unix philosophy. Use short code snippets only when they clearly illustrate a concept. Do **not** add extra commentary outside the defined sections.