fix(skill): use portable Agent Skills frontmatter (#57)

Encode `allowed-tools` as a space-delimited string and add the spec's `compatibility` field, so SKILL.md validates against the Agent Skills spec and `gh skill publish`. Rule set is unchanged: Claude Code's parser is paren-aware, so the string resolves to the same 13 permissions as the previous YAML list.

Also condenses the README's multi-agent install guidance to one paragraph.

Author: vdmkotai

README.md

@@ -148,7 +148,7 @@ Happens consistently after the third click."
 npx skills add drogers0/gh-image
 ```
 
-The open [Agent Skills standard](https://agentskills.io/clients) is supported by **Claude Code**, **OpenAI Codex**, **Cursor**, **GitHub Copilot**, and [many more](https://agentskills.io/clients). The skill checks that this extension is present (asking you to install it if not), runs the upload, and embeds the resulting `user-attachments` URL into a PR, issue, or comment. It never installs anything on your behalf.
+The installer detects your local agents automatically; add `--agent claude-code codex opencode --global` to install for several at once. The open [Agent Skills standard](https://agentskills.io/clients) is supported by **Claude Code**, **OpenAI Codex**, **OpenCode**, **Cursor**, **GitHub Copilot**, and [many more](https://agentskills.io/clients). The skill checks that this extension is present (asking you to install it if not), runs the upload, and embeds the resulting `user-attachments` URL into a PR, issue, or comment. It never installs anything on your behalf.
 
 ## Who's using gh-image
 

skills/github-image-upload/SKILL.md

@@ -10,22 +10,14 @@ description: >-
   document or attach files to changes on GitHub. Powered by the `gh-image` gh CLI
   extension.
 license: MIT
+compatibility: Requires the GitHub CLI (`gh`), network access to GitHub, and the `drogers0/gh-image` extension v1.1.0 or newer.
 # gh pr/issue edit and comment are listed for hosts that check each pipeline stage;
 # whole-string matchers resolve them via the leading printf.
-allowed-tools:
-  - Glob
-  - Bash(gh auth status)
-  - Bash(gh extension list:*)
-  - Bash(gh image:*)
-  - Bash(gh pr view:*)
-  - Bash(gh pr edit:*)
-  - Bash(gh pr comment:*)
-  - Bash(gh issue view:*)
-  - Bash(gh issue edit:*)
-  - Bash(gh issue comment:*)
-  - Bash(printf:*)
-  - Bash(grep:*)
-  - Bash(cat:*)
+allowed-tools: >-
+  Glob Bash(gh auth status) Bash(gh extension list:*) Bash(gh image:*)
+  Bash(gh pr view:*) Bash(gh pr edit:*) Bash(gh pr comment:*)
+  Bash(gh issue view:*) Bash(gh issue edit:*) Bash(gh issue comment:*)
+  Bash(printf:*) Bash(grep:*) Bash(cat:*)
 ---
 
 # Upload images and files to GitHub (gh-image)