ACP: plugin skills aren't available in ACP sessions, though the plugin's agents are #5269

Describe the bug

In ACP mode (copilot --acp --stdio), skills from enabled plugins aren't available in a session. The plugin itself loads, since the agent config option offers its agents. But none of its skills:

• are in available_commands_update, so an ACP client can't offer them as slash commands;
• show in /skills list sent over ACP, which lists only project and personal skills;
• reach the model: asked to list its skills, it names only the personal one.

Outside ACP, with the same COPILOT_HOME and folder, the skills are there: copilot skill list lists them under "Plugin skills", and in copilot -p the model lists them.

Affected version

GitHub Copilot CLI 1.0.104 (also seen on 1.0.92)

Steps to reproduce the behavior

1. Enable plugins that have skills, and add one personal skill as a control. Here, two plugins from local marketplace directories, which copilot plugin list shows under "Live Plugins". Between them they have 12 agents and 36 skills.
2. copilot skill list lists the 36 under "Plugin skills".
3. Start copilot --acp --stdio, initialize, and call session/new with a cwd.
4. The agent config option offers the 12 plugin agents (<plugin>:<agent>).
5. available_commands_update has 33 commands: the 32 built-in commands and the personal skill. None of the plugin skills are there.
6. Send /skills list as a session/prompt. It lists only the personal skill ("Found 1 skill").
7. Send Without using any tools, list the names of all the skills in your available skills list, comma-separated, and nothing else. The reply names only the personal skill.
8. In the same folder, run copilot -p with the same text. The model lists the personal skill and the plugin skills.

Expected behavior

An ACP session has the same skills as the interactive CLI and prompt mode, plugin skills included:

• they're offered in available_commands_update, as 1.0.40 did for skills ("Skills are available as slash commands in ACP clients, matching the CLI experience");
• /skills list lists them;
• the model can use them.

Additional context

• Windows 11, x86_64, driven from a Node ACP client, in a scratch COPILOT_HOME with no extensions installed.
• Project and personal skills work over ACP. /skills reload picks up new ones and sends available_commands_update.
• Related: Plugin skills not included in available_skills for main agent — only builtin skills visible #2753 (plugin skills missing from <available_skills> in interactive sessions). There, a comment ties it to extensions that call joinSession(). Here no extension is installed, and prompt mode isn't affected.