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 in the shared bar layout so both pinned and inline terminal surfaces behave consistently
• update the live bar immediately while preserving complete raw panel state and terminal-height safety
• document the setting and cover config, overflow, viewport, and runtime-update behavior

Test plan

• uv run --frozen pytest -q (12,458 passed, 81 skipped, 1 xpassed)
• uv run --frozen ruff check <changed Python files>
• uv run --frozen ruff format --check <changed Python files>