feat: add URL support to COG_SCHEMA_GEN_TOOL, remove silent legacy fallback
- COG_SCHEMA_GEN_TOOL now accepts https:// URLs in addition to local paths;
downloaded binaries are cached under ~/.cache/cog/bin/
- Remove binary-not-found fallback in canUseStaticSchemaGen — for SDK >= 0.17.0,
the embedded binary is always available; missing binary is now a hard error
instead of silently falling back to deleted cog.command
- Gate PATH and dist/ fallbacks behind isDev so production builds only use
the embedded binary (or explicit COG_SCHEMA_GEN_TOOL override)