feat: M6 complete — summarize strategy + tool result persistence

SummarizeStrategy: calls LLM to condense older messages into a
summary, preserving key decisions, file changes, tool outputs.
Falls back to truncation on failure. Keeps 6 recent messages.

Tool result persistence: outputs >50K chars saved to disk at
.gnoma/sessions/tool-results/{id}.txt with 2K preview inline.

TUI: /compact command for manual compaction, /clear now resets
engine history. Summarize strategy used by default (with
truncation fallback).

internal/engine/loop.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"fmt"
 
+	gnomactx "somegit.dev/Owlibou/gnoma/internal/context"
 	"somegit.dev/Owlibou/gnoma/internal/message"
 	"somegit.dev/Owlibou/gnoma/internal/permission"
 	"somegit.dev/Owlibou/gnoma/internal/provider"
@@ -248,6 +249,12 @@ func (e *Engine) executeTools(ctx context.Context, calls []message.ToolCall, cb
 			output = e.cfg.Firewall.ScanToolResult(output)
 		}
 
+		// Persist large results to disk
+		if persisted, ok := gnomactx.PersistLargeResult(output, call.ID, ".gnoma/sessions"); ok {
+			e.logger.Debug("tool result persisted to disk", "name", call.Name, "size", len(output))
+			output = persisted
+		}
+
 		// Emit tool result event for the UI
 		if cb != nil {
 			cb(stream.Event{