fix: harden generate color data extraction

[reobin]

the generate job could lose all extracted color data when one group used a numeric rgb value, and larger repositories could exceed extractor.nvim's per-run limit.

the root causes live in extractor.nvim: vimcolorschemes/extractor.nvim#7 makes hexCode always a #RRGGBB string, raises errors instead of returning silently, drops the 100-colorscheme cap, and encodes an empty extraction as {}. this pr hardens the worker around that contract, so it merges after extractor.nvim#7.

• extracts in deterministic batches, sized to stay under the per-run timeout
• filters out built-in colorschemes before extraction, with stable ordering
• runs extractor calls under pcall and turns failures into a nonzero nvim exit, streaming stderr
• parses extractor output, failing the run on empty or malformed content (the extractor always writes at least {})
• drops the autocmd-based extraction from vim/init.lua
• adds coverage for parsing, batching, and filtering behavior