feat(skills): add inference.sh skill (terminal-based, no custom tools) (#1686)

Add inference.sh as a built-in skill that uses the terminal tool to
run infsh CLI commands. No custom tools or tool registration — the
skill teaches the agent how to use the infsh binary via terminal.

Covers 150+ AI apps: image gen (FLUX, Reve, Seedream), video (Veo,
Wan, Seedance), LLMs, search (Tavily, Exa), 3D, avatars, and more.

Includes reference docs for authentication, app discovery, running
apps, and CLI command reference.

Based on PR #1021 by @okaris, reworked as a skill-only integration.

Co-authored-by: okaris <okaris@users.noreply.github.com>

skills/inference-sh/cli/references/authentication.md

@@ -0,0 +1,59 @@
+# Authentication & Setup
+
+## Install the CLI
+
+```bash
+curl -fsSL https://cli.inference.sh | sh
+```
+
+## Login
+
+```bash
+infsh login
+```
+
+This opens a browser for authentication. After login, credentials are stored locally.
+
+## Check Authentication
+
+```bash
+infsh me
+```
+
+Shows your user info if authenticated.
+
+## Environment Variable
+
+For CI/CD or scripts, set your API key:
+
+```bash
+export INFSH_API_KEY=your-api-key
+```
+
+The environment variable overrides the config file.
+
+## Update CLI
+
+```bash
+infsh update
+```
+
+Or reinstall:
+
+```bash
+curl -fsSL https://cli.inference.sh | sh
+```
+
+## Troubleshooting
+
+| Error | Solution |
+|-------|----------|
+| "not authenticated" | Run `infsh login` |
+| "command not found" | Reinstall CLI or add to PATH |
+| "API key invalid" | Check `INFSH_API_KEY` or re-login |
+
+## Documentation
+
+- [CLI Setup](https://inference.sh/docs/extend/cli-setup) - Complete CLI installation guide
+- [API Authentication](https://inference.sh/docs/api/authentication) - API key management
+- [Secrets](https://inference.sh/docs/secrets/overview) - Managing credentials