guardrails: context-budget blocks can dead-end broad edits into background instead of team

[terisuke]

Summary\nWhen the guardrail profile blocks a broad edit after the source-read budget is exhausted, the resulting remediation path can dead-end. The current context-budget error says to narrow scope or delegate before editing, but the parallel implementation policy separately requires the tool. Meanwhile, write-capable runs are still allowed even when the team gate is active, so the agent can choose background workers, fail to satisfy the gate, and end up stuck.\n\n## Repro sketch\n1. In a guarded repo with commits, send a broad multi-file implementation request so the team plugin activates the parallel gate.\n2. Read enough unique source files to trip the context budget.\n3. Attempt an edit/write.\n4. Observe the budget block says to narrow scope or delegate.\n5. Launch write-capable workers instead of .\n6. The gate is still unmet, so later direct mutations remain blocked.\n\n## Expected\n- When the parallel gate is active, write-capable runs should not be accepted as a substitute for .\n- The context-budget block should tell the user/agent to use the tool explicitly when that gate is active.\n\n## Actual\n- The remediation message is ambiguous under the parallel policy.\n- Write-capable runs are allowed even though they do not satisfy the gate.\n\n## Proposed fix\n- Share parallel-gate state so the guardrail plugin can tailor the context-budget error message.\n- Reject write-capable runs while the team gate is active, with a clear error that points to .\n- Add regression coverage for the combined context-budget + parallel-policy flow.\n