feat(harness): support per-subagent compaction config

[March-77]

Summary

Fixes #2325.

Declarative subagents could only use their own default compaction behavior because SubagentDeclaration had no compaction option and the declared-subagent factory did not propagate the parent setting.

This change:

• adds SubagentDeclaration.Builder.compaction(CompactionConfig) for per-subagent configuration;
• applies declaration-level compaction before inherited parent settings;
• preserves parent disabled/configured compaction when no declaration override is present;
• adds regression coverage for overriding a disabled parent and inheriting a disabled parent.

User impact

Callers can configure a declaratively registered subagent with an independent compaction strategy while other subagents continue to inherit the parent behavior.

Tests

• mvn -pl agentscope-harness -am -Dtest="HarnessAgentTest,SubagentDeclarationPhaseATest" -Dsurefire.failIfNoSpecifiedTests=false test (44 tests passed)
• Spotless check passed as part of the Maven build.

Compatibility and risk

The new builder method is additive. Existing declarations keep inheritance semantics, so no migration is required. The change is limited to local declared-subagent construction; remote task declarations are unaffected.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...cope/harness/agent/HarnessAgentBuilderSupport.java	88.88%	0 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!

[oss-maintainer]

LGTM. Looks good.

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