[Bug] ACP mode never sends available_commands_update, so slash commands and skills cannot be completed

[AndriyPytel]

Affected area

Coding agent and CLI (ACP mode)

What happened?

prime-agent --mode acp never sends an available_commands_update notification, so an ACP client receives no command list for the session. The same session in the TUI completes every slash command, skill, and prompt template; over ACP a user has to type /skill:<name> blind, with no way to discover what is loaded.

ACP has a first-class available_commands_update session update for exactly this, and clients use it to drive completion.

Steps to reproduce

1. Run prime-agent --mode acp.
2. Send initialize, then session/new.
3. Watch the session/update notifications.
4. No available_commands_update ever arrives; the client's command list stays empty.

Expected behavior

After session/new, the agent advertises the commands a prompt turn can actually execute: the session slash commands (compact, refine, goal, autonomous), plus everything connection.getCommands() returns — skills as skill:<name>, prompt templates, and extension commands.

Commands whose execution is not "session" (/model, /settings, /export) should stay out: they open a TUI selector and have no headless behavior, so completing to them would only insert prompt text.

Prime Agent version

0.7.1 (also reproduced on main at 7787f07)

Environment

macOS, prime-agent --mode acp driven over stdio by an ACP client

Additional context

Tracked as #1306. A focused patch with a test that drives a real @agentclientprotocol/sdk client over an in-memory duplex is up as #1308 (it fails on main with "no available_commands_update yet"); npm run check passes.

Opening this Discussion because the new contribution process routes intake here — happy to take the patch forward if it is in scope, or to drop it if you would rather solve command discovery differently.

[AndriyPytel]

@sethkarten flagging this to you directly since you wrote ACP mode, and this category is new enough that nothing here has been seen yet.

Context for why I care: I drive Prime Agent through ACP every day rather than through the TUI, and this is the thing I trip over most. The commands are all there in the session, the client simply is never told about them, so anything loaded as a skill is invisible unless you already know its name.

The fix turned out to be small, so I wrote it up as #1308 with a test that fails on main. Completely fine if you would rather solve command discovery another way, or not expose it over ACP at all — in that case say so and I will close the PR. I mostly wanted the problem on the record.