feat(docker): Docker session mode foundation (types, storage, tmux builders)

Phase 0-2 of the Docker cases feature (docs/docker-cases-plan.md). Docker is a
LOCATION OVERLAY on cases (not a 6th SessionMode), mirroring the remote-SSH
feature: a local tmux pane runs `docker exec -it` into a durable in-container
tmux server. The container is per-CASE, so multiple sessions share it.

- types: DockerHost/DockerCase/SessionDocker + docker? on SessionState/MuxSession
- src/docker-hosts.ts: storage, toSessionDocker, pure buildDockerBaseArgs/
  buildDockerCreateArgs (cap-drop, no-new-privileges, --pull=never, mem==swap,
  never privileged/socket), containerApiUrl, hostGatewayAlias, config-hash,
  credential-mount resolution, daemon probes (VITEST no-op)
- schemas: DockerHostSchema + DockerCaseLinkSchema (NO_SHELL_META guards)
- tmux-manager: buildDockerLaunchCommand (image-check -> ensure -> start -> exec,
  resume-aware), buildDockerKillCommand (in-container tmux only, multi-session
  safe), stop/remove; wired into createSession/respawnPane/killSession
- 40 unit tests (docker-hosts + docker-exec-options), typecheck clean

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/mux-interface.ts

@@ -18,6 +18,7 @@ import type {
   EffortLevel,
   GeminiConfig,
   SessionRemote,
+  SessionDocker,
 } from './types.js';
 
 /**
@@ -36,6 +37,8 @@ export interface MuxSession {
   workingDir: string;
   /** Remote execution metadata for local tmux sessions wrapping SSH */
   remote?: SessionRemote;
+  /** Docker execution metadata for local tmux sessions wrapping `docker exec` */
+  docker?: SessionDocker;
   /** Session mode */
   mode: SessionMode;
   /** Whether webserver is attached to this session */
@@ -79,6 +82,8 @@ export interface CreateSessionOptions {
   historyLimit?: number;
   /** Remote execution metadata for local tmux sessions wrapping SSH */
   remote?: SessionRemote;
+  /** Docker execution metadata for local tmux sessions wrapping `docker exec` */
+  docker?: SessionDocker;
 }
 
 /** Options for respawning a dead pane. */
@@ -103,6 +108,8 @@ export interface RespawnPaneOptions {
   historyLimit?: number;
   /** Remote execution metadata for local tmux sessions wrapping SSH */
   remote?: SessionRemote;
+  /** Docker execution metadata for local tmux sessions wrapping `docker exec` */
+  docker?: SessionDocker;
 }
 
 /** Options for pane buffer capture (COD-47 full-history mode). */