fix(harness): enforce plan mode for subagents

[RainYuY]

Summary

• Install Plan Mode enforcement for automatically built general-purpose and declared subagents.
• Propagate the parent's Plan Mode configuration, including shell access and the plan file directory.
• Resynchronize Plan Mode state before spawning, reusing, or messaging persistent subagents.
• Add regression tests covering Plan Mode propagation and write-tool restrictions.

Motivation

Plan Mode enforcement is provided by PlanModeMiddleware, rather than by the shared planActive state alone.

Previously, automatically built subagents inherited the parent's Plan Mode state but did not install the corresponding manager and middleware. As a result, tools such as write_file and edit_file could bypass the read-only boundary while the parent agent was in Plan Mode.

The parent agent was unaffected because its Plan Mode middleware was installed during its own build process.

Testing

mvn -nsu -pl agentscope-harness -am \
  -DskipITs \
  -Dtest=PlanModeSubagentPropagationTest,AgentSpawnToolPlanModeTest \
  -Dsurefire.failIfNoSpecifiedTests=false \
  test

[codecov]

Codecov Report

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../agentscope/harness/agent/tool/AgentSpawnTool.java	88.88%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[RainYuY]

@jujn @chickenlj PTAL