refactor: simplify task tool subagent filtering

[malhashemi]

Summary

Small cleanup to align the task tool's filtering approach with how skill.ts handles it, plus simplification of the userInvokedAgents mechanism.

Self-contained filtering:

• Move subagent filtering into task.ts itself (using ctx?.agent from #7042)
• Remove the post-hoc regeneration in prompt.ts

Simplify bypass logic:

• Replace userInvokedAgents array with bypassAgentCheck boolean
• Only check the current turn's user message (not all messages in session)
• Prevents bypass from persisting across the entire session

Test cleanup:

• Remove redundant filterSubagents tests (already covered by PermissionNext.evaluate tests)

Changes

• task.ts: Filter during description generation + simplify permission check
• prompt.ts: Remove regeneration block + use bypassAgentCheck boolean
• test: Remove redundant tests (~130 lines)

No functional changes to permission behavior - just cleaner code!