feat(harness): add projectWorkspace for filesystem tools

[kongweiguang]

Summary

Revives and retargets the closed #1348 work onto the current main branch.

• Adds HarnessAgent.Builder.projectWorkspace(Path) so built-in file tools can operate on a project directory that is separate from the Harness state workspace.
• Keeps workspace(...) responsible for AGENTS.md, memory, sessions, skills, workspace context, and other Harness state.
• Uses a virtual LocalFilesystem rooted at projectWorkspace for file tools without the per-user namespace.
• Preserves sandbox-backed file tool behavior by default when projectWorkspace is not configured.
• Propagates projectWorkspace to shared/general-purpose subagents so their file tools use the same project directory.

Why

The old PR targeted the deleted harness branch, so it was closed without merging. This reapplies the feature against current main and adapts it to the newer HarnessAgent/subagent structure.

Validation

• mvn -pl agentscope-harness spotless:apply
• mvn -pl agentscope-harness -am -Dtest=HarnessAgentTest test

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...java/io/agentscope/harness/agent/HarnessAgent.java	81.25%	2 Missing and 1 partial ⚠️

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