[Bug]:compaction summary leaks host filesystem path into the agent prompt (invalid inside Docker sandbox)

**Describe the bug**

When compaction is enabled and the agent runs in a Docker sandbox, the injected summary message leaks a **host-side filesystem path** to the original conversation history instead of a sandbox-visible path.

With `CompactionConfig.offloadBeforeCompact` at its default (`true`), `MemoryFlushManager#resolveOffloadPath` returns the host-local session-JSONL path via `workspaceManager.resolveSessionContextFile(...).toString()`, and `ConversationCompactor#buildSummaryMessage` (~L441) embeds it verbatim into the agent-facing summary text:

> *The full conversation history has been saved to `.agentscope\docker-compaction\workspace\alice\agents\compaction-agent\sessions\alice-session-id.jsonl` should you need to refer back to it for details.*

This path does not exist inside the container (where `pwd` is `/workspace`). On a Windows host the path also contains backslashes, which are valid filename characters — not separators — on Linux, so any attempt by the agent to "refer back" to it fails. The statement is factually wrong and can mislead the agent.

**To Reproduce**

Steps to reproduce the behavior:

1. Build a `HarnessAgent` with a `DockerFilesystemSpec` and a `CompactionConfig` that triggers compaction (default `offloadBeforeCompact(true)`):

```java
HarnessAgent agent = HarnessAgent.builder()
.name("compaction-agent")
.model(model)
.workspace(Paths.get(".agentscope/docker-compaction/workspace"))
.stateStore(new JsonFileAgentStateStore(Paths.get(".agentscope/docker-compaction/state")))
.compaction(CompactionConfig.builder()
.triggerMessages(10)
.keepMessages(3)
.build())
.filesystem(new DockerFilesystemSpec()
.image("ubuntu:24.04")
.snapshotSpec(new LocalSnapshotSpec(Paths.get(".agentscope/docker-compaction/snapshot"))))
.build();
RuntimeContext ctx = RuntimeContext.builder().userId("alice").sessionId("alice-session-id").build();
agent.call(new UserMessage("Create an agents.md file declaring your name is coder ..."), ctx).block();
```

2. The single `agent.call` must exercise enough tool calls for the message history to exceed `triggerMessages`. The prompt above triggers 5 tool rounds — `list_files` ×2, `execute("pwd && ls -la")`, `write_file`, `read_file` — i.e. 11 messages, which trips compaction.
3. Inspect the injected summary USER message (`name = __compaction_summary__`) in the session JSONL. It contains the host path shown above; inside the container that path does not exist.

**Expected behavior**

The compaction summary should not present a host-local filesystem path to an agent executing inside a sandbox. Either omit the path from the summary text (the offloaded archive is for operator/external use, not for the agent to `cat` inside the container), or expose a path that is actually resolvable within the sandbox namespace (if a host→sandbox path translation is available).

**Error messages**

No exception or stack trace — this is a silent semantic defect. The observable signal is the incorrect path inside the summary message. If the agent subsequently tries to read it (e.g. `read_file(".agentscope\docker-compaction\...")`), the tool call fails with `Empty or not a directory` / file-not-found because the path does not exist in the container.

**Environment (please complete the following information)**

- AgentScope-Java Version: 2.0.0
- Java Version: 17
- OS: Windows (host); sandbox is an `ubuntu:24.04` Docker container

**Additional context**

- Workaround: `CompactionConfig.builder().offloadBeforeCompact(false)` makes `buildSummaryMessage` take the path-less `else` branch. But this also disables message offload, so it is a workaround rather than a fix.
- The bug is mostly invisible under the Local (non-sandbox) filesystem — the path separator is correct and the file lives in the same filesystem. The Docker sandbox amplifies it (path does not exist + `\` is not a separator on Linux), which is likely why it went unnoticed.
- Relevant source (2.0.0): `agentscope-harness/.../memory/compaction/ConversationCompactor.java` (~L441, `buildSummaryMessage`) and `agentscope-harness/.../memory/MemoryFlushManager.java` (~L194, `resolveOffloadPath`).