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

AGENTS.md

@@ -33,14 +33,36 @@ You supply the music theory: a per-bar chord chart and a melody.
 }
 ```
 
+## Indexing convention (read this once, then never guess)
+
+- **Bars are 0-based everywhere**: bar 0 is the first bar of the chord chart.
+- **`loop.end_bar` is EXCLUSIVE**: `{"start_bar": 4, "end_bar": 20}` loops
+  bars 4..19 — 16 bars. `start_bar` is inclusive.
+- The melody must fit the chart: `melody_start_bar + (number of '|' bars)
+  <= len(chords)`. Bars past the melody (e.g. chords 20–23 above) are an
+  outro the engine plays before jumping back to `start_bar`.
+- `fills_at` entries are 0-based bar numbers, `0 <= bar < len(chords)`.
+
+## Validation behaviour
+
+`make_tune.py` validates the whole spec up front and reports **every**
+problem in one go (missing keys, unknown style, bad chord quality,
+`bpm <= 0`, out-of-range loop/fill bars, melody longer than the chart,
+unsafe `name`) — no raw `KeyError`s, no silently wrong output. Fields a
+style can't use (e.g. `counter` for `n64_ambient`, `fills_at` for styles
+without drums) print `WARNING:` to stderr and are skipped, never dropped
+in silence. `name` must match `[A-Za-z0-9][A-Za-z0-9._-]*` — it becomes
+`midi/<name>.mid`, so no `/`, no `..`.
+
 ## Score grammar (melody strings)
 
 - Tokens: `PITCH-dur-vel` — `A4-8-90` (A4 eighth, vel 90). Durations:
   `1` whole, `2` half, `4` quarter (default), `8` eighth, `16` sixteenth.
 - Rests: `r4`, `r8`, `r16`, `r2`, `r1`.
-- `|` separates bars. **If you use `|`, every bar must total exactly 4/4**
-  — the parser raises `ValueError: bar N totals X ticks` and tells you which
-  bar is wrong. Use it; it is the single most useful bug-catcher here.
+- `|` separates bars. **If you use `|`, every bar — including the final
+  one — must total exactly 4/4** — the parser raises
+  `ValueError: bar N totals X ticks` and tells you which bar is wrong.
+  Use it; it is the single most useful bug-catcher here.
 - Pitches: letter + optional `#`/`b` + octave, e.g. `Bb3`, `F#5`.
 
 ## Verification loop (do this, don't skip it)