feat: configure subagent panel row limit

[GregKinne]

Summary

• add a curated subagent_panel_max_rows setting with /set validation
• use 0 (the default) for automatic terminal-height sizing and positive values for an explicit rendered-row ceiling
• apply the ceiling consistently to pinned and inline terminal surfaces
• update the live bar immediately while preserving complete raw panel state and terminal-height safety
• document the setting and add automated regression coverage

Validation so far

• automated test suite: 12,458 passed, 81 skipped, 1 xpassed
• Ruff checks passed
• formatting checks passed

Still needed

• manual testing in Code Puppy before considering any upstream submission