fix(convert): make antigravity date_added deterministic (#608)

convert_antigravity() stamped `date_added: '${TODAY}'` (the convert-run date), so
every regeneration produced different bytes for every antigravity skill — churning
the gitignored output and blocking byte-reproducible rendering downstream (the app
can't implement a renderer for output it can't reproduce).

Replace ${TODAY} with a fixed constant (ANTIGRAVITY_DATE_ADDED="2026-03-08",
matching the documented example in integrations/antigravity/README.md). The field
stays (it's part of the Antigravity frontmatter format); it's just stable now.

Verified: two consecutive `convert.sh --tool antigravity` runs produce a
byte-identical SKILL.md (same sha), and no convert-run date appears in output.

This unblocks the app from rendering antigravity (format `antigravity-skill` in
tools.json) once it implements that renderer.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/convert.sh

@@ -63,6 +63,12 @@ REPO_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
 OUT_DIR="$REPO_ROOT/integrations"
 TODAY="$(date +%Y-%m-%d)"
 
+# Static date stamped into Antigravity skill frontmatter. Deliberately fixed (NOT
+# the convert-run date): a per-run TODAY made every regeneration produce different
+# bytes, which churns the gitignored output and breaks byte-reproducible rendering
+# downstream. Matches the documented example in integrations/antigravity/README.md.
+ANTIGRAVITY_DATE_ADDED="2026-03-08"
+
 # Shared helpers (get_field, get_body, slugify, ...)
 # shellcheck source=lib.sh
 . "$SCRIPT_DIR/lib.sh"
@@ -125,7 +131,7 @@ name: ${slug}
 description: ${description}
 risk: low
 source: community
-date_added: '${TODAY}'
+date_added: '${ANTIGRAVITY_DATE_ADDED}'
 ---
 ${body}
 HEREDOC