demo-recorder: codify VHS hardening — foreground execution, nonzero-artifact verification, Wait+Line caveat

## Observed

Two failure instances of VHS-based demo recording across two waves of the same pipeline:
1. A per-AC demo produced a 0-byte GIF when VHS ran as a background task under system load (race); a foreground re-run succeeded.
2. A wave-close reel dispatch stalled entirely (600s no-progress watchdog kill) with VHS in the background; the retry with explicit "run VHS in FOREGROUND, verify GIF >0 bytes before committing" instructions succeeded.

The successful retry also documented two environment quirks worth carrying in the agent prompt: `Wait+Line` is non-functional in VHS 0.11.0 (times out immediately; use Sleep durations), and VHS `Type` argument parsing rejects some absolute paths in quoted strings (workaround: backtick quoting / short symlink paths).

## Engine gap

The demo-recorder agent prompt does not carry these operational rules; each orchestrator session re-discovers them and injects them manually into dispatch prompts. Recording failures cost a full agent dispatch each (the stalled attempt burned its whole timebox).

## Suggested fix

Add to the demo-recorder agent prompt / skill: (a) always run VHS in the foreground; (b) verify every produced artifact is nonzero-size before committing, re-record in foreground on failure; (c) known-quirks note for Wait+Line (VHS 0.11.0) and Type-argument path parsing; (d) prefer Sleep-based pacing when Wait+Line is unavailable.

## Evidence

Sessions 2026-07-05/06, private consumer project: one 0-byte GIF race (caught by the recorder's own size check, foreground re-run succeeded) and one full-dispatch stall (600s watchdog), retry successful under foreground discipline.