fix(contracts): restore snake_case field names in the exec-channel union

[Yambr]

Restores three handshake field names to the schema's universal snake_case convention: env_vars → env, boundPid → bound_pid, supports_trace → supports_traces.

boundPid was the only camelCase property in the schema. Both language bindings already declare these fields as env / bound_pid / supports_traces and carried per-field rename attributes to compensate; the existing prose in docs/future-architecture also references supports_traces. Frozen-contract changes ride in dedicated PRs.

The companion ocu-sandbox PR updates the embedded schema copies, fixtures, and both bindings; the byte-identity gate there verifies against this file.

🤖 Generated with Claude Code

Summary by CodeRabbit

Chores

• Updated Exec WebSocket control protocol schema with normalized property names: env_vars → env and boundPid → bound_pid.
• Renamed trace-negotiation flag from supports_trace to supports_traces across connection capabilities and trace-event references for consistency.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro Plus

Run ID: 344f0459-d338-4295-b605-67a1c5c48b4d

📥 Commits

Reviewing files that changed from the base of the PR and between 93ee911 and c3e0ed6.

📒 Files selected for processing (1)

• contracts/exec/exec-channel.schema.json

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Walkthrough

The exec-channel WebSocket protocol schema receives coordinated naming updates: CreateProcess properties are renamed for consistency (env_vars → env, boundPid → bound_pid), and the trace-negotiation capability field is renamed from supports_trace to supports_traces with matching updates to all comments and documentation references.

Changes

Schema Naming Standardization

Layer / File(s)	Summary
CreateProcess property renames contracts/exec/exec-channel.schema.json	CreateProcess schema properties are renamed from env_vars to env and from boundPid to bound_pid for naming convention consistency.
Trace capability field rename and documentation contracts/exec/exec-channel.schema.json	The boolean trace-negotiation field in ConnectionCapabilities is renamed from supports_trace to supports_traces. All referential comments in server-side and client-side TraceEvent definitions and open-questions documentation are updated to use the new field name.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: restoring snake_case naming conventions in the exec-channel schema by renaming env_vars, boundPid, and supports_trace fields.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/exec-channel-snake-case

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