fix: preserve long run activity and large previews

Long Runtime sessions could silently stop retaining tool and Subagent activity, truncate Run history details, and reject workspace previews over 256 KB. Run history now persists in Main SQLite without the former fixed record or detail caps, surfaces possible loss from legacy caches, and renders long histories in batches.

Local and remote workspace previews now load UTF-8-safe 256 KiB pages on demand instead of rejecting larger text files. The legacy migration, clear-data path, IPC contracts, tests, and feature documentation are updated with the resulting behavior.

Release note: 长时间 Runtime 运行现在会完整保留工具和 Subagent 活动；运行记录迁移到 SQLite，并提示旧版可能已丢失的内容。超过 256KB 的本地及远程文本文件可通过分页面预览继续加载。

FEATURES.md

@@ -86,8 +86,9 @@ otherwise.
   the same project and different projects can run in parallel. Request-scoped
   dynamic MCP tools remain isolated through default wildcard disablement and
   explicit current-request enablement. OpenCode and Continue runs do not stop
-  at a fixed tool-call or activity count, and exceeding local display capture
-  capacity truncates the displayed copy without cancelling the native task.
+  at a fixed tool-call or activity count, and every observed tool and Subagent
+  activity remains in the local conversation instead of being discarded while
+  the native task continues.
   Interactive questions are answered only by foreground conversations; scheduled tasks, remote channels,
   delegated work, and other background runs fail immediately with guidance to
   rerun in the foreground
@@ -393,10 +394,14 @@ otherwise.
   activity, and independent artifact files. Ordinary chat responses remain
   only in the conversation and are no longer copied into Artifacts; existing
   duplicate chat Markdown is hidden from the artifact list but not physically
-  deleted. Token usage is grouped by Runtime and model and normalizes the
-  different OpenAI-compatible and Anthropic Messages cache-reporting semantics
-  when showing cache hit rate. Activity is grouped by conversation and
-  collapsed by default so long histories do not fill the page.
+  deleted. Run history is stored in Main SQLite without the previous 500-item,
+  4,000-character, or 2 MB Renderer limits; the page renders it in batches,
+  and a persistent warning remains when a migrated legacy cache may already
+  have lost older records or detail. Token usage is grouped by Runtime and
+  model and normalizes the different OpenAI-compatible and Anthropic Messages
+  cache-reporting semantics when showing cache hit rate. Activity is grouped
+  by conversation and collapsed by default so long histories do not fill the
+  page.
 - [x] **Task and custom-task experience**: Each product-level Task belongs to
   one Conversation, while one Conversation can contain multiple Tasks. The
   left conversation list exposes Task children through a leading expand