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

README.md

@@ -44,15 +44,37 @@ changes. CI tags every change as a `vX.Y.Z` release (see
 
 ## Wired harnesses
 
-Phase 1 (current) wires **Claude Code** only:
+| Harness | Target path | Notes |
+|---|---|---|
+| **Claude Code (global)** | `~/.claude/skills/<name>` | Visible in every Claude Code session on the host. |
+| **Claude Code (per-repo)** | `<repo>/.claude/skills/<name>` | Created when `install` is invoked inside a git repo (not the skills repo itself). Gitignored globally via `**/.claude/skills/`. |
+| **Crush** | `~/.config/crush/skills/<name>` | Charmbracelet Crush — successor to `opencode-ai/opencode` (archived). |
+| **Antigravity** | `~/.gemini/antigravity/skills/<name>` | Google Antigravity (VS Code extension). Our `SKILL.md` frontmatter (`name` + `description`) is already in the format Antigravity expects, so symlinks suffice — no manifest translation. |
 
-- Global: `~/.claude/skills/<name>` → `~/.local/share/skills/<name>`
-- Per-repo: `<repo>/.claude/skills/<name>` (when invoked from a git
-  repo) → same
+### Not wired (and why)
 
-Phase 2 will add: Crush, opencode, antigravity, gitea-resident agents
-(cobalt-dingo, agentsquad). See `mathias/infra` issue `infra#62`
-addendum for the roadmap.
+- **opencode** (`opencode-ai/opencode`): archive notice; succeeded by Crush.
+  Its only "skill"-like surface — Custom Commands at `~/.config/opencode/commands/` —
+  is for user-facing prompt templates, not system-level instructions.
+  Misaligned with how we use skills. Skipped.
+- **gitea-resident agents** (cobalt-dingo, agentsquad): they consume
+  skills via their containing project's `.claude/skills` directory
+  (populated by the per-repo wirer when run from the host), or via the
+  brain MCP. No special target needed.
+
+### Per-host env-var overrides
+
+Each target path can be overridden by setting the matching env var before
+running `install.sh`:
+
+| Env var | Default |
+|---|---|
+| `SKILLS_REPO_URL` | `https://gitea.d-ma.be/mathias/skills.git` |
+| `SKILLS_REF` | `main` (set to e.g. `v0.1.0` to pin a release) |
+| `SKILLS_CHECKOUT_DIR` | `$HOME/.local/share/skills` |
+| `CLAUDE_SKILLS_DIR` | `$HOME/.claude/skills` |
+| `CRUSH_SKILLS_DIR` | `$HOME/.config/crush/skills` |
+| `ANTIGRAVITY_SKILLS_DIR` | `$HOME/.gemini/antigravity/skills` |
 
 ## Versioning