Harden for agent users: spec validation, key-sig accidentals, N64 chord qualities

- make_tune: validate_spec() reports every spec problem in one run
  (missing keys, unknown style, bad qualities, bpm<=0, out-of-range
  loop/fills, melody longer than chart, unsafe name) — no raw KeyErrors
- make_tune: key_sig_for handles accidentals in minor and major keys,
  any case (Bb minor, F# minor, bb major); real spellings kept (C# major)
- make_tune: n64 pad uses QUALITIES table (dim/aug/sus now correct)
- make_tune: WARNING on stderr for fields a style can't use (counter
  for non-DOS, fills_at for n64) instead of silent drops
- make_tune: name sanitised ([A-Za-z0-9][A-Za-z0-9._-]*), no traversal
- make_tune: render errors actionable (timidity presence check, stderr
  surfaced, sys.executable instead of bare python3)
- songkit: parse_score lints the FINAL bar too (was unchecked); this
  caught a real 5-beat bar in 01_service_station_2am COUNTER_A (fixed)
- build_all.sh: dependency + soundfont checks, removes stale midi/wav
  outputs with no current source, fails if nothing was produced
- smoke_test: runs every spec, checks loop marker tick positions,
  feeds malformed bars to the linter, covers validation + key sigs
- AGENTS.md: documents 0-based bars and exclusive end_bar, validation
  behaviour; SKILL.md added so the repo loads as an agent skill

tunes/01_service_station_2am.py

@@ -55,7 +55,7 @@ COUNTER_A = """
 r2 D5-8 F5-8 E5-4 | r2 F5-8 D5-8 Bb4-4 |
 r2 G5-8 F5-8 D5-4 | r2 E5-8 G5-8 F5-8 E5-8 |
 r4 A5-8 G5-8 F5-4 A5-4 | r4 F5-8 A5-8 D5-4 F5-4 |
-r2 C6-4 Bb5-4 | r2 F5-4 D5-4 r4
+r2 C6-4 Bb5-4 | r2 F5-4 D5-4
 """