Add JLCPCB PCBA conventions skill; fix broken pygerber invocation in gerbers skill

[stefangolas]

��Add JLCPCB PCBA conventions skill; fix broken pygerber invocation in gerbers skill

skills/jlcpcb: documents JLCPCB's placement and BOM conventions for
Standard PCBA with through-hole assembly:

• every BOM designator requires a CPL row, including through-hole parts;
  the BOM checker rejects lines with no CPL row and drops them from
  assembly
• CPL Mid X / Mid Y is the pad-field centre, not the footprint origin
  (KiCad parks a connector's origin on pin 1, while JLC centres each part
  model on its CPL point)
• rotation is read from the side the part is mounted on, so bottom-side
  parts carry (360 - rot) % 360 while KiCad measures CCW from the top
• one CPL row per physical part
• model-zero corrections are per package family
• connector gender must be confirmed on the LCSC page category line
• an audit-file pattern: the exporter emits a side-by-side record of
  footprint origin, pad-field centre, and rotation in both conventions,
  and a test replays the shipped CPL against it

skills/gerbers: the documented pygerber invocation
('pygerber gerber convert png ...') does not work on any released
pygerber. Verified against the release history:

• 1.0.0 / 1.1.0 (2021): flat CLI, no subcommands; 'pygerber --save
  (--pillow|--blender) [--yaml|--json|--toml]'. No -o, no -d, no style
  names; -s is the save path.
• 2.0.0rc1 / rc2: no CLI entry point (no console script, no main).
• 2.0.0 .. 2.4.3: the subcommands are 'raster-2d' and 'render'.
  Replace with the working 2.x invocation ('pygerber raster-2d ...
  --dpi N -s copper_alpha'), note that --dpi is dots per inch, and add a
  minimum-version constraint (1.x ships a different, specfile-driven CLI).

[i2cjak]

skills must be written by a human to prevent ensloppa.

the JLC skill should be a script under BOM or export instead imo, this is what I use to convert KiCad position files to JLCPCB compliant files.

[stefangolas]

Fair enough ill remove the jlcpcb part but I think the pygerbers one is valid