feat: wire Crush + Antigravity harnesses

Phase 2 of mathias/skills extraction. Adds two new per-tool wirers
alongside Claude Code:

- Crush — ~/.config/crush/skills/<name>/SKILL.md (charmbracelet/crush,
  successor to opencode-ai/opencode which is archived)
- Antigravity — ~/.gemini/antigravity/skills/<name>/SKILL.md (Google
  VS Code extension). Our SKILL.md frontmatter (name + description)
  is already in the format Antigravity expects — no manifest
  translation step needed.

Both wirers added to Taskfile.yml (install:crush + install:antigravity)
and install.sh (wire_crush + wire_antigravity). The aggregate `install`
target now calls all four targets (claude:global, claude:repo, crush,
antigravity). Idempotent symlinks, same shape as the Claude Code wirer.

Per-host env overrides documented in README: SKILLS_REF for tag pinning,
CRUSH_SKILLS_DIR + ANTIGRAVITY_SKILLS_DIR for non-default paths.

Skipped:
- opencode (opencode-ai/opencode): archived, succeeded by Crush; its
  Custom Commands surface is user prompt templates, not system skills
- gitea-resident agents: consume via per-repo .claude/skills or brain
  MCP, no special target needed

Bump-Type: minor

install.sh

@@ -17,6 +17,8 @@ REPO_URL="${SKILLS_REPO_URL:-https://gitea.d-ma.be/mathias/skills.git}"
 REF="${SKILLS_REF:-main}"
 CHECKOUT_DIR="${SKILLS_CHECKOUT_DIR:-$HOME/.local/share/skills}"
 CLAUDE_GLOBAL_DIR="${CLAUDE_SKILLS_DIR:-$HOME/.claude/skills}"
+CRUSH_DIR="${CRUSH_SKILLS_DIR:-$HOME/.config/crush/skills}"
+ANTIGRAVITY_DIR="${ANTIGRAVITY_SKILLS_DIR:-$HOME/.gemini/antigravity/skills}"
 
 log() { printf '[skills] %s\n' "$*"; }
 
@@ -69,6 +71,28 @@ wire_claude_global() {
   done < <(list_skills)
 }
 
+wire_crush() {
+  # Crush reads skills from ~/.config/crush/skills/<name>/SKILL.md.
+  # Pre-creating the dir is cheap even when Crush isn't installed.
+  mkdir -p "$CRUSH_DIR"
+  while IFS= read -r skill; do
+    [ -n "$skill" ] || continue
+    link_skill "$CRUSH_DIR" "$skill"
+  done < <(list_skills)
+}
+
+wire_antigravity() {
+  # Antigravity (Google's VS Code extension) reads global skills from
+  # ~/.gemini/antigravity/skills/<name>/SKILL.md. Our SKILL.md files
+  # already carry the required `name` + `description` YAML frontmatter,
+  # so symlinks are sufficient — no manifest translation step.
+  mkdir -p "$ANTIGRAVITY_DIR"
+  while IFS= read -r skill; do
+    [ -n "$skill" ] || continue
+    link_skill "$ANTIGRAVITY_DIR" "$skill"
+  done < <(list_skills)
+}
+
 wire_claude_repo() {
   # Only wire per-repo when invoked from inside a git repo and it isn't
   # the skills repo itself.
@@ -92,6 +116,8 @@ main() {
   ensure_checkout
   wire_claude_global
   wire_claude_repo
+  wire_crush
+  wire_antigravity
   log "done — $(list_skills | wc -l | tr -d ' ') skill(s) wired at ref=$REF"
 }