feat(harness): add projectWorkspace for filesystem tools

[kongweiguang]

Summary

Retargets the closed #1528 work so the source branch is kongweiguang:main instead of a temporary feature branch.

• Adds HarnessAgent.Builder.projectWorkspace(Path) so built-in file tools can operate on a project directory 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.

Validation

• mvn -pl agentscope-harness -am -Dtest=HarnessAgentTest test
• GitHub Actions: Check License, Check Module Sync, Ubuntu build, Windows build all passed.
• Codecov patch: all modified and coverable lines are covered by tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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