feat(core): enforce workspace mutation ordering [2 of 5]

[fcoury-oai]

Why

A directory mutation cannot execute like an ordinary parallel tool call. Calls after it must observe the updated cwd and permission context, and they must not run against stale state if the mutation fails.

What Changed

• Add an explicit ToolExecutionPolicy with parallel and barrier-and-cancel-suffix modes.
• Execute preceding calls before a barrier, run the barrier exclusively, and release later calls only after success.
• Emit dependency_cancelled outputs for calls skipped behind a failed mutation barrier.
• Preserve ordinary parallel execution between barriers.
• Share dependency-failure construction and test setup across the ordering paths.

How to Test

1. Submit a batch with ordinary calls before and after a successful barrier.
2. Confirm calls before the barrier complete first and suffix calls start only after success.
3. Repeat with a failed barrier.
4. Confirm suffix calls are not dispatched and receive dependency_cancelled outputs.

Targeted tests:

• failed_barrier_cancels_suffix_without_dispatching_it
• The focused tests in core/src/tools/parallel.rs

Stack

1. #25336 runtime workspace state
2. feat(core): enforce workspace mutation ordering [2 of 5] #25339(feat(core): enforce workspace mutation ordering [2 of 5] #25339) ordered mutation barriers (this PR)
3. #25334 model workspace tools
4. #25338 approval projection
5. #25335 TUI workspace commands

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