fix(subturn): inherit runtime tool registry for spawned subagents

[Alix-007]

Summary

Fixes #1713.

This PR keeps spawned/subagent child turns aligned with the parent turn's runtime tool registry, instead of pre-snapshotting tools into SubTurnConfig.Tools.

• spawn / subagent now leave SubTurnConfig.Tools unset (nil) so child turns inherit at execution time from the parent turn context.
• spawnSubTurn now prioritizes cloning baseAgent.Tools when available, preserving registry semantics (including hidden tools and TTL gating).
• Explicit cfg.Tools is only used as a fallback when the parent registry is unavailable and the explicit slice is non-empty.
• Empty explicit tool slices no longer clear inherited tools.

Why

The bug report describes spawned subagents missing tool definitions in provider requests. The robust fix is to inherit from the live parent registry so child turns receive the same tool visibility and runtime additions as the parent, while keeping hidden/TTL behavior intact.

Validation

Focused tests were added/updated to assert provider-level tool definitions and edge cases:

• TestSpawnSubTurn_EmptyExplicitToolsStillInheritParentRegistry
• TestSpawnSubTurn_InheritsRuntimeAddedTools
• TestSpawnSubTurn_PreservesHiddenTTLSemantics
• TestSpawnSubTurn_UsesExplicitToolsWhenParentRegistryUnavailable
• TestSpawnTool_Execute_LeavesToolsUnsetForRuntimeInheritance
• TestSubagentTool_Execute_LeavesToolsUnsetForRuntimeInheritance

Commands run:

• go test ./pkg/tools -run 'TestSpawnTool_Execute_(ValidTask|LeavesToolsUnsetForRuntimeInheritance)|TestSubagentTool_Execute_(Success|LeavesToolsUnsetForRuntimeInheritance)' -count=1\n- go test ./pkg/agent -run 'TestSpawnSubTurn_(EmptyExplicitToolsStillInheritParentRegistry|InheritsRuntimeAddedTools|PreservesHiddenTTLSemantics|UsesExplicitToolsWhenParentRegistryUnavailable)' -count=1\n- go test ./pkg/tools -run 'TestSpawnTool_|TestSubagentTool_' -count=1\n- go test ./pkg/agent -run 'TestSpawnSubTurn_' -count=1