refactor: extract SnapshotController so the runtime no longer brokers /undo

[dgageot]

Fixes #2701.

The runtime used to know about snapshots: it carried snapshotsEnabled, exposed UndoLastSnapshot / ListSnapshots / ResetSnapshot as methods on LocalRuntime, and the TUI commands reached snapshots through that runtime. This PR moves snapshots into a SnapshotController returned by builtins.RegisterSnapshot, threaded into both the runtime (as an AutoInjector) and the App.

What changed

• pkg/hooks/builtins/snapshot.go \u2014 new SnapshotController interface (Enabled, UndoLast, List, Reset) and RegisterSnapshot(r, enabled) (SnapshotController, error) entry point that registers the snapshot builtin and returns a controller. The controller's AutoInject mounts the snapshot hook on session/turn boundaries when enabled.
• pkg/hooks/builtins/builtins.go \u2014 added a generic AutoInjector interface; Register no longer wires the snapshot builtin and AgentDefaults.Snapshot is gone (the snapshot block in ApplyAgentDefaults moved onto the controller).
• pkg/runtime/snapshot.go \u2014 deleted.
• pkg/runtime/runtime.go / hooks.go \u2014 removed WithSnapshots, snapshotsEnabled, and the snapshot methods. New WithHooksRegistry lets the embedder ship a registry with snapshot pre-registered, and WithAutoInjector plumbs in any AutoInjector. buildHooksExecutors runs the registered injectors.
• pkg/app/ \u2014 App gains a snapshotController via WithSnapshotController(c). UndoLastSnapshot / ListSnapshots / ResetSnapshot delegate directly to the controller, resolving cwd from the session (with os.Getwd fallback).
• cmd/root/run.go \u2014 a small snapshotRuntimeOpts helper builds a registry, calls RegisterSnapshot, and returns the runtime opts plus the controller; buildAppOpts threads the controller into the App. The session spawner does the same so each spawned session has independent snapshot state.
• lint/hook_builtins_registered.go \u2014 exempts snapshot.go from the "must be registered in builtins.go" check since it has its own RegisterSnapshot entry point.

Acceptance criteria

• pkg/runtime/snapshot.go is gone.
• LocalRuntime exposes no snapshot-specific methods.
• TUI commands drive the controller directly (through the App, no longer through the runtime).
• WithSnapshots is removed from runtime.Opt; configuration moves to the controller.

Validation

• task build \u2014 ok
• go test ./... \u2014 ok
• task lint \u2014 0 issues

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