agentHost: support running tasks in remote workspaces

Adds a pluggable task-runner registry (ISessionTaskRunnerRegistry) so the agents window can dispatch session tasks through different runtimes:

- WorkbenchSessionTaskRunner (priority 0) preserves today's ITaskService path for local file workspaces.

- AgentHostSessionTaskRunner (priority 100) opens an agent-host terminal via IAgentHostTerminalService and sendText's the resolved command, so tasks now work in remote agent-host sessions where there's no extension host.

Also adds a WorktreeCreatedTaskDispatcher contribution that runs `runOptions.runOn === 'worktreeCreated'` tasks once per session; the cloud-agent provider opts out via a new ISessionCapabilities.runsWorktreeCreatedTasks flag because its server-side host already runs those during provisioning.

resolveTaskCommand handles command/args (with POSIX shell quoting honoring strong/weak/escape CommandString metadata), OS overrides, npm scripts, and dependsOn chains (sequence -> '&&', parallel -> backgrounded subshells with trailing wait).

Fixes #312052

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>