agent: save session after each tool round for mid-turn crash recovery

Adds SaveSession callback to agentConfig, called after every state.update()
in the run loop. On restart, the session JSON contains all completed tool
rounds; the model continues naturally from current history.

SIGTERM is already handled in olliesrv via InterruptAll + Shutdown, which
cancels all session contexts and lets the loop exit cleanly after the
current state.update(). Worst-case (SIGKILL) loses one tool round, not
the whole turn.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/agent/core.go

@@ -777,6 +777,7 @@ func (s *agent) executeTurn(ctx context.Context, input string, handler EventHand
 			"result":     result,
 		}, s.log)
 	}
+	s.cfg.SaveSession = func() { s.saveSession() }
 	s.cfg.AutoCompact = func(ctx context.Context) {
 		if ctx.Err() != nil || s.session == nil {
 			return

pkg/agent/loop.go

@@ -24,6 +24,7 @@ type agentConfig struct {
 	GenerationParams backend.GenerationParams
 	PopInject        func() string                                                      // returns and clears pending inject, or ""
 	AutoCompact      func(ctx context.Context)                                           // called after each tool round; may compact in-place
+	SaveSession      func()                                                             // called after each state.update(); persists mid-turn progress
 	PreTool          func(ctx context.Context, name string, args json.RawMessage) HookResult  // called before each tool; exit 2 blocks execution
 	PostTool         func(ctx context.Context, name string, args json.RawMessage, result string) HookResult // called after each tool; exit 0 appends, exit 2 replaces result
 }
@@ -127,6 +128,9 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 				})
 			}
 			state.update(msg, results)
+			if cfg.SaveSession != nil {
+				cfg.SaveSession()
+			}
 			return fmt.Errorf("step %d: stream interrupted: %w", step, ctx.Err())
 		}
 
@@ -252,6 +256,9 @@ func run(ctx context.Context, cfg agentConfig, state state) error {
 		}
 
 		state.update(msg, results)
+		if cfg.SaveSession != nil {
+			cfg.SaveSession()
+		}
 
 		if interrupted {
 			return ctx.Err()