agent: add ToolCallCount to Core interface; wire tcct accumulator

Adds a monotonically increasing tool-call counter (tcct) to the agent.

- atomic.Int64 toolCallCount on the agent struct
- ToolCallCount() int64 on the Core interface
- IncrToolCallCount func() int64 on agentConfig, wired in executeTurn
- Incremented in execOne after the pre-tool hook returns (blocked calls
  don't count) and before cfg.Exec is called (timeout/error still counts)

The counter never resets. Consumers use modulo arithmetic:
  tcct % 10 == 0  ->  fires every 10 actual executions

The primary consumer is ollie-9p, which exposes tcct as a read-only
9P file that pre_tool / post_tool hooks can inspect via cat.

pkg/agent/core.go

@@ -217,6 +217,7 @@ type agent struct {
 	cwd             string
 	startupMessages  []string
 	currentAction   atomic.Pointer[actionHandle]
+	toolCallCount   atomic.Int64
 	fifo            PromptFIFO
 	pendingInject   atomic.Pointer[string]
 	mu              sync.RWMutex
@@ -231,6 +232,13 @@ type agent struct {
 	auditLog        *olog.Logger
 }
 
+// ToolCallCount returns the total number of tool calls executed in this
+// session. The counter is monotonically increasing and never resets.
+// Blocked calls (pre-tool hook exit 2) are not counted.
+func (s *agent) ToolCallCount() int64 {
+	return s.toolCallCount.Load()
+}
+
 // SetEnv stores a session-scoped variable and propagates it to the execute server.
 func (s *agent) SetEnv(key, value string) {
 	s.envMu.Lock()
@@ -916,6 +924,9 @@ func (s *agent) executeTurn(ctx context.Context, input string, handler EventHand
 			"result":     result,
 		}, s.log)
 	}
+	s.cfg.IncrToolCallCount = func() int64 {
+		return s.toolCallCount.Add(1)
+	}
 	s.cfg.SaveSession = func() { s.saveSession() }
 	s.cfg.AutoCompact = func(ctx context.Context) {
 		if ctx.Err() != nil || s.session == nil {

pkg/agent/types.go

@@ -121,6 +121,14 @@ type Core interface {
 	// Supported fields: WatchState, WatchUsage, WatchCtxSz, WatchCWD.
 	WaitChange(ctx context.Context, field, current string) (string, bool)
 
+	// ToolCallCount returns the total number of tool calls executed in this
+	// session since the agent was created. The counter is monotonically
+	// increasing and never resets. Blocked calls (pre-tool hook exit 2) are
+	// not counted. Timed-out or erroring calls are counted because execution
+	// was attempted. Use modulo arithmetic in hooks to fire every N calls:
+	//   [ $(($(cat tcct) % 10)) -eq 0 ] && ...
+	ToolCallCount() int64
+
 	// Close releases resources associated with the session, including its
 	// temporary directory under /tmp/ollie/.
 	Close()