Document that power symbols are global while labels are sheet-scoped in instanced sheets

[mixelpixx]

Found in Chris's end-to-end run on Konnect v0.11.0 / KiCad 10.0 / Windows 11 / Claude Desktop (QuadNode PCIe accelerator card). Verified against current main unless noted.

batch_connect_to_net emits sheet-local labels. In a sheet instanced 4x that is correct for per-node nets but silently wrong for rails meant to be global: using power:+5V symbols briefly shorted all four nodes' independent 5V rails together. One sentence in the tool description and the schematic skill would have prevented it.

[mixelpixx]

Closed by #483 (merged as eddf131, exact head 901f9f9, ten checks green). Acceptance: both tool descriptions (connect_to_net, batch_connect_to_net) and the schematic skill now state that plain labels are sheet-local while power symbols and global labels are one net across every sheet and instance, with the repeated-sheet exception written into rule 6. Documentation only; no behaviour change. Verified the two KiCad facts match the QuadNode observation that raised this.