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

Taskfile.yml

@@ -12,6 +12,11 @@ vars:
   REPO_URL: 'https://gitea.d-ma.be/mathias/skills.git'
   CHECKOUT_DIR: '{{.HOME}}/.local/share/skills'
   CLAUDE_GLOBAL_DIR: '{{.HOME}}/.claude/skills'
+  CRUSH_DIR: '{{.HOME}}/.config/crush/skills'
+  ANTIGRAVITY_DIR: '{{.HOME}}/.gemini/antigravity/skills'
+  # Anything at the repo root that is NOT a skill directory. Both
+  # Taskfile + install.sh share this exclusion regex.
+  NON_SKILL_ENTRIES: '^(Taskfile.yml|install.sh|README.md|SKILLS_INDEX.md|.gitea|.git$)$'
 
 tasks:
 
@@ -26,17 +31,19 @@ tasks:
     silent: true
 
   install:
-    desc: 'Wire skills into every harness detected on this host (Claude Code today, more in phase 2).'
+    desc: 'Wire skills into every supported harness (Claude Code, Crush, Antigravity).'
     cmds:
       - task: install:claude:global
       - task: install:claude:repo
+      - task: install:crush
+      - task: install:antigravity
 
   install:claude:global:
     desc: 'Per-skill symlinks under ~/.claude/skills/<name> — visible in every Claude Code project on this host.'
     cmds:
       - mkdir -p {{.CLAUDE_GLOBAL_DIR}}
       - |
-        for skill in $(ls -1 "{{.TASKFILE_DIR}}" | grep -Ev "^(Taskfile.yml|install.sh|README.md|SKILLS_INDEX.md|.gitea|.git$)$"); do
+        for skill in $(ls -1 "{{.TASKFILE_DIR}}" | grep -Ev "{{.NON_SKILL_ENTRIES}}"); do
           target="{{.TASKFILE_DIR}}/${skill}"
           link="{{.CLAUDE_GLOBAL_DIR}}/${skill}"
           if [ -L "$link" ] || [ -e "$link" ]; then
@@ -63,7 +70,7 @@ tasks:
         fi
         target_dir="$repo/.claude/skills"
         mkdir -p "$target_dir"
-        for skill in $(ls -1 "{{.TASKFILE_DIR}}" | grep -Ev "^(Taskfile.yml|install.sh|README.md|SKILLS_INDEX.md|.gitea|.git$)$"); do
+        for skill in $(ls -1 "{{.TASKFILE_DIR}}" | grep -Ev "{{.NON_SKILL_ENTRIES}}"); do
           target="{{.TASKFILE_DIR}}/${skill}"
           link="${target_dir}/${skill}"
           if [ -L "$link" ] || [ -e "$link" ]; then
@@ -77,6 +84,44 @@ tasks:
           echo "linked $link → $target"
         done
 
+  install:crush:
+    desc: 'Per-skill symlinks under ~/.config/crush/skills/<name> — visible in every Crush session on this host.'
+    cmds:
+      - mkdir -p {{.CRUSH_DIR}}
+      - |
+        for skill in $(ls -1 "{{.TASKFILE_DIR}}" | grep -Ev "{{.NON_SKILL_ENTRIES}}"); do
+          target="{{.TASKFILE_DIR}}/${skill}"
+          link="{{.CRUSH_DIR}}/${skill}"
+          if [ -L "$link" ] || [ -e "$link" ]; then
+            current=$(readlink "$link" 2>/dev/null || true)
+            if [ "$current" = "$target" ]; then
+              continue
+            fi
+            rm -rf "$link"
+          fi
+          ln -s "$target" "$link"
+          echo "linked $link → $target"
+        done
+
+  install:antigravity:
+    desc: 'Per-skill symlinks under ~/.gemini/antigravity/skills/<name>. SKILL.md frontmatter (name + description) is already antigravity-compatible.'
+    cmds:
+      - mkdir -p {{.ANTIGRAVITY_DIR}}
+      - |
+        for skill in $(ls -1 "{{.TASKFILE_DIR}}" | grep -Ev "{{.NON_SKILL_ENTRIES}}"); do
+          target="{{.TASKFILE_DIR}}/${skill}"
+          link="{{.ANTIGRAVITY_DIR}}/${skill}"
+          if [ -L "$link" ] || [ -e "$link" ]; then
+            current=$(readlink "$link" 2>/dev/null || true)
+            if [ "$current" = "$target" ]; then
+              continue
+            fi
+            rm -rf "$link"
+          fi
+          ln -s "$target" "$link"
+          echo "linked $link → $target"
+        done
+
   update:
     desc: 'git pull the canonical checkout then re-run install.'
     cmds: