feat: complete M1 — core engine with Mistral provider

Mistral provider adapter with streaming, tool calls (single-chunk
pattern), stop reason inference, model listing, capabilities, and
JSON output support.

Tool system: bash (7 security checks, shell alias harvesting for
bash/zsh/fish), file ops (read, write, edit, glob, grep, ls).
Alias harvesting collects 300+ aliases from user's shell config.

Engine agentic loop: stream → tool execution → re-query → until
done. Tool gating on model capabilities. Max turns safety limit.

CLI pipe mode: echo "prompt" | gnoma streams response to stdout.
Flags: --provider, --model, --system, --api-key, --max-turns,
--verbose, --version.

Provider interface expanded: Models(), DefaultModel(), Capabilities
(ToolUse, JSONOutput, Vision, Thinking, ContextWindow, MaxOutput),
ResponseFormat with JSON schema support.

Live verified: text streaming + tool calling with devstral-small.
117 tests across 8 packages, 10MB binary.

internal/engine/callback.go

@@ -0,0 +1,7 @@
+package engine
+
+import "somegit.dev/Owlibou/gnoma/internal/stream"
+
+// Callback receives streaming events for real-time UI updates.
+// Called synchronously on the engine goroutine for each event.
+type Callback func(stream.Event)