Rebuild the generic packs, add shell and sysadmin, fix short race passages

The packs were not proper. An audit found 86 of 253 items (33%) carrying an
attribution that just restated the category - prose *about* a topic with an
invented source, which is the same fault the movies pack had. Six of thirteen
packs were mostly that: technology had 15 items and one distinct attribution.

- science, technology, history, nature and business are now sourced quotes
  with real attributions: Feynman, Curie, Hopper, Dijkstra, Lincoln, Carson,
  Drucker, Goodhart. 82 items, all attributed to a person or a work.
- general is original factual prose, so it now carries no attribution at all
  rather than claiming "General knowledge" as a source. merge_packs no longer
  invents one from the pack's filename.
- Four explanatory passages in philosophy lost their "Philosophy" attribution
  for the same reason.
- One duplicated passage removed.

Generic attributions: 86/253 before, 0/349 now.

New technical packs
- shell: 24 awk, sed and pipeline drills, each explaining what the line does
  - field splitting, associative arrays, !seen[$0]++, process substitution,
  xargs -0, strict mode.
- sysadmin: 24 operational one-liners across systemd, disk, processes,
  network, permissions, SSH, backup and containers.
- programming grew to 37 and hacking to 30, with git bisect, window
  functions, EXPLAIN ANALYZE, certificate transparency and capability audits.
- All 115 technical drills carry an explanation.

Race passage length
- Multiplayer drew from the same pool as single player, so a race could land
  on a 22-character quote and be over before anyone had their hands in
  position. Races now require 120 characters; the filter is applied when the
  pool is loaded, not to the packs, since a short quote is fine to type alone.

For reference, TypeRacer organises by difficulty and language rather than
topic: one default English pool of ~11,900 texts plus per-language universes
and specials (accuracy, repeat, easytexts, anime). Their scale comes from user
submission with moderation, which is still the feature this does not have.

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