feat: extract tool executor and support termination

[augmentcode]

Summary

Implements Agent Runtime Optimization Wave 4.

• Extracts tool scheduling, hooks, permissions, plan-mode checks, output bounding, deduplication, and tool events into DefaultToolExecutor.
• Keeps Agent as the composition/session-writeback layer.
• Adds ToolBatchOutcome (Continue / Terminate) to the turn runtime contract.
• Allows tools to opt into terminating the current tool batch via Tool::terminate_batch().
• Prevents the automatic follow-up model call when a batch terminates successfully.
• Preserves existing tool-result, mode-change, session writeback, and ACP behavior.

Validation

• cargo check --workspace --locked
• cargo test -p zene-turn -p zene-tools -p zene-core --lib
• git diff --check
• rustfmt check for the new executor

All targeted tests pass. Existing unrelated unused-import warnings remain.

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Pull Request opened by Augment Code | View session