ACP mode: fork, session resume, and model config gaps

[rawizhere]

I maintain bb-plugin-prime-agent-acp, a plugin for bb (an IDE for agent threads) that runs the prime-agent binary as an ACP provider. The integration is in decent shape: subagents, goal state, compaction, refinements and autonomous counters all come through _meta and render natively in bb's UI.

What doesn't work is the other direction. bb's provider bridge sends standard ACP requests that the ACP mode has no handler for, and there's nothing I can do about it client-side, since the agent is the one that has to answer. Three of these block features I'd like to ship:

Fork. Forking a thread in bb issues session/fork {sessionId, cwd, mcpServers} and fails with does not advertise session/fork support. ACP mode doesn't advertise sessionCapabilities.fork and has no handler, and I couldn't find any PR for it. Open PR #1389 already adds a daemon command that copies a slice of the conversation into a new session file without touching the original, which is probably the backend an ACP session/fork handler needs.

Resume. loadSession: false and no session/load, so every restarted thread starts with blank in-agent history. PR #1600 fixed exactly this (the description mentions bb threads), and PR #1116 went further with session/list on top of listSavedSessions(). Both closed without review.

Model picking. session/new returns only a sessionId, so bb shows an empty model picker and hides reasoning selection. PR #2105 added selectors to session/new plus session/set_config_option with config_option_update notifications. The agent side already has getModelCatalog(), setModel() and setThinkingLevel(); they just aren't exposed.

Smaller things: available_commands_update is never sent, so slash commands and skills are invisible to ACP clients (formerly #1306 / #1308). Context usage already has an open discussion (#1356) and a complete implementation in PR #1351 — is that one still planned?

As far as I can tell, all of this only needs code in modes/acp/. fork, switchSession, getModelCatalog and friends already exist on the daemon connection. I get why the PR queue got closed, and I'm not asking anyone to reopen old PRs. Just asking whether any of this is of interest. If something lands I'm happy to test it against a real client build.