Add Ink skills and symlink installer

AGENTS.md

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+# AGENTS.md
+
+`ink-skills` contains reusable Ink-specific Agent Skills and one symlink installer.
+
+## Layout
+
+- `skills/<name>/SKILL.md` — one narrowly triggered on-demand behavior.
+- `bin/ink-skills` — dependency-free installer; stdout is TSV, diagnostics stderr.
+- `test/install-smoke.sh` — installer contract smoke.
+
+## Rules
+
+- A skill owns reusable judgment, never runtime policy or repeatable mechanics.
+- Ink source and `REQUIREMENTS.md` own host behavior; `toolset` owns external executables.
+- Keep skill names lowercase and hyphenated; directory and frontmatter name must match.
+- The installer only creates symlinks and must refuse collisions. Do not add a registry,
+  network calls, package-manager dependency, prompts, copies, or hidden state.
+- Run `sh -n bin/ink-skills` and `sh test/install-smoke.sh` after changes.