fix: reclaim idle remote runtimes and retire replaced agents

Remote tasks could leave idle Runtime processes behind, and replaced Agent
installations lacked a drain-and-exit lifecycle. Keep the current Agent
resident, let displaced Agents finish existing work and reconnect for its
results, then retire them without restarting obsolete installations.

Reclaim Runtime processes when their last active task finishes, preserve
pending questions and shared active tasks, and restore conversation history
when the next turn starts. Remove the Remote Projects technical preview label
from settings, errors, tests, and current documentation; the feature remains
disabled by default.

Validation recorded during implementation: 213 lifecycle tests and 28 focused
UI/IPC tests passed, as did SettingsPanel and lint. Linux x64 real-Host checks
used 8 provider requests. Broader tests and final typecheck encountered
unrelated concurrent changes; details and coverage limits are documented.

Release note: 远程任务结束后自动释放空闲运行进程，下一轮对话恢复历史；更新后的旧 Agent 等待已有任务完成再退出，当前 Agent 保持常驻。远程项目移除“技术预览”标识，仍默认关闭。

FEATURES.md

@@ -266,8 +266,8 @@ records are listed separately and do not introduce another feature status.
   first; invalid paths fail at actual launch. On managed Linux ARM Hosts,
   Runtime activation reuses the registry and manifest verified during setup
   instead of rehashing or rechecking the complete OpenCode binary.
-- [x] **Managed SSH OpenCode and Continue (technical preview)**: Controlled by the
-  separate Remote Projects (Technical Preview) tab under Settings / Platform
+- [x] **Managed SSH OpenCode and Continue**: Controlled by the
+  separate Remote Projects tab under Settings / Platform
   Features and disabled by default. Disabling it does not affect local
   projects, ordinary desktop capabilities, or desktop releases. When enabled,
   users can manage SSH Hosts with pinned Host Keys, browse bounded remote
@@ -279,6 +279,10 @@ records are listed separately and do not introduce another feature status.
   provider/tool rounds, Runtime processes, a stable model ledger, and a bounded
   semantic transcript over a private Unix socket and ACP v5. Work continues on
   the Host after Desktop exit, network loss, or local-process termination.
+  The current Agent may remain resident; a superseded Agent drains active work
+  before exiting. Idle Runtime processes are reclaimed after tasks finish,
+  while shared processes retain other active tasks, including pending questions.
+  Subsequent turns recreate the Runtime and restore conversation history.
   Reconnection attaches only to the original controller, binding, and
   operation, then atomically merges provenance into the original conversation
   without resending the Prompt. Model profiles and API keys enter Agent memory