docs: update docs (#43)

- macOS, Linux badge and AI suggestions showcase in README
- Update user guide and docs for development

docs/dev/README.md

@@ -0,0 +1,15 @@
+# Iris developer & architecture documentation
+
+This directory contains code architecture guides, engine design notes, and development instructions for Iris contributors.
+
+## Table of contents
+
+- [Development workflow & runner](development.md): Environment setup, building, testing, linters, and modular `just` runner (`justfiles/`)
+- [AI engine architecture](ai.md): Internal AI completion system (`internal/ai`), providers, prompt design, and debounce strategy
+- [Scoring & frecency engine](scoring.md): Scoring mechanics (`internal/scoring`), frecency calculations, context rules, and skeleton extraction
+- [Overlay & rendering pipeline](overlay.md): PTY overlay positioning, cell width calculation, and terminal rendering
+- [PTY bridge & keystroke wrapper](root.md): Keystroke interception loop, raw mode PTY handling, and IPC scanner
+- [Spec & completion engine](spec.md): Static specifications, priority-based flag gating, and Cobra `__complete` dynamic completion
+- [File & path generator](filegen.md): File system traversal, extension filtering, and directory slash preservation
+- [History provider](history.md): Shell history indexing, search algorithms, and caching
+- [Auto updater](updater.md): Release tracking, version comparison, and atomic binary updates