docs: update docs (#43)

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

docs/dev/ai.md

@@ -0,0 +1,29 @@
+# AI engine architecture (`internal/ai/`)
+
+The AI subsystem provides real-time, context-aware command completions powered by cloud or local LLM providers (Groq, Ollama).
+
+## Core components
+
+- `internal/ai/client.go`: Defines the `Client` interface and handles HTTP requests to OpenAI-compatible chat completion endpoints.
+- `internal/ai/env.go`: Captures runtime context snapshots (`EnvSnapshot`) including CWD, previous command, exit code, and recent history.
+- `internal/ai/prompts.go`: Constructs system and user prompts optimized for shell command completion.
+- `root/wrapper.go`: Manages async debounce timers, request cancellation (`context.WithCancel`), and ghost text injection.
+
+## Provider interface
+
+All providers use a unified HTTP pattern matching OpenAI's `/v1/chat/completions` format:
+
+```go
+type Client interface {
+    Suggest(ctx context.Context, prompt string, env *EnvSnapshot, currentCmd string) (*AISuggestion, error)
+}
+```
+
+## Request lifecycle
+
+1. User types in the prompt buffer.
+2. `root/wrapper.go` triggers a debounce timer (`debounce_ms`, default 500ms).
+3. If typing continues, previous in-flight contexts are cancelled (`aiCancel()`).
+4. An `EnvSnapshot` is created capturing CWD, last executed command, and up to 3 recent history entries.
+5. Provider sends an HTTP POST request with structured JSON payload.
+6. Response is parsed and rendered as inline ghost text via `overlay.InjectAISuggestion()`.