docs: document skill install methods (npx skills, gh skill, plugin, npm)

Now that the canonical skill lives at the repo root, npx skills add and
gh skill install resolve it. List all install paths in the README.

Author: theoephraim

README.md

@@ -102,17 +102,31 @@ bumpy publish   # pack and publish, create git tags, push tags, and create GitHu
 
 ## AI Integration
 
-Bumpy ships with an [agent skill](https://github.com/dmno-dev/bumpy/blob/main/skills/add-change/SKILL.md) that teaches LLMs how to create bump files.
+Bumpy ships with an [agent skill](https://github.com/dmno-dev/bumpy/blob/main/skills/add-change/SKILL.md) that teaches LLMs how to create bump files. It teaches the AI to examine git changes, identify affected packages, choose bump levels, and create bump files with `bumpy add` — and to keep existing bump files up to date as work continues on a branch, updating packages, bump levels, and summaries to reflect the final state of changes.
 
-For Claude Code, install it as a plugin:
+### Installing the skill
 
-```bash
-claude plugin install @varlock/bumpy
-```
+Pick whichever fits your setup:
+
+- **[`skills`](https://github.com/vercel-labs/skills) (any agent — recommended)** — works with Claude Code, Cursor, OpenCode, Codex, and others:
+
+  ```bash
+  npx skills add dmno-dev/bumpy     # update with: npx skills update
+  ```
+
+- **GitHub CLI** (v2.90+):
+
+  ```bash
+  gh skill install dmno-dev/bumpy add-change     # update with: gh skill update add-change
+  ```
+
+- **Claude Code plugin:**
 
-The skill is also bundled in the published npm package, so once bumpy is installed it lives at `node_modules/@varlock/bumpy/skills/add-change/SKILL.md` — version-pinned to your installed bumpy. Agents and tools that support the [SKILL.md](https://github.com/dmno-dev/bumpy/blob/main/skills/add-change/SKILL.md) format can reference it directly.
+  ```bash
+  claude plugin install @varlock/bumpy
+  ```
 
-The skill teaches the AI to examine git changes, identify affected packages, choose bump levels, and create bump files with `bumpy add`. It also instructs the AI to keep existing bump files up to date as work continues on a branch - updating packages, bump levels, and summaries to reflect the final state of changes.
+- **npm-bundled** — the skill ships inside the published package, so once `@varlock/bumpy` is installed it lives at `node_modules/@varlock/bumpy/skills/add-change/SKILL.md`, version-pinned to your installed bumpy. Point any tool that consumes a `SKILL.md` path at it directly.
 
 ## Documentation