Agent roster + ad-hoc ACP agents: configure any ACP-speaking CLI, not just known presets

[liam-russell]

Problem

Two hard limits in the current agent config (packages/types/src/agents.ts):

1. One agent, total. AgentConfig is a single { command, args, mode } — "only one agent is active at a time; there is no roster." The fan-out flow (Task fan-out: create worker worktrees with pre-seeded agent sessions in one action #137) wants per-subtask agent choice (a strong planner dispatching cheaper workers), which is impossible with a single slot.
2. ACP is allowlisted. Integrated (chat) mode is only offered when the command matches ACP_PRESET_TOKENS (claude, gemini, codex, kiro, cursor-agent). ACP is an open stdio protocol — obscure or internal agents (Snowflake CoCo, company-internal wrappers, brand-new tools) may speak it perfectly well, but SproutGit forces them into raw-terminal mode.

Proposal

• Agent roster: a list of named agents in Settings. Existing presets stay as quick-add buttons; the on-demand ACP adapter install flow (acp-adapters.ts) keeps working for the known presets.
• Ad-hoc entries: { name, command, args, env, mode, acp: boolean } — the user asserts "this command speaks ACP" instead of the token allowlist deciding. Per-agent env vars cover API keys and base URLs (which also lets any CLI agent run against OpenRouter or a proxy without global env pollution).
• Test button: extend agent:test to do a real ACP initialize handshake and report the agent's name/version/capabilities — instant feedback that an ad-hoc integration works before it's used in anger.
• Selection: a default agent, overridable per launch and per fan-out subtask (Task fan-out: create worker worktrees with pre-seeded agent sessions in one action #137).
• Import/export: agent definitions as JSON so a team can share a working config for an obscure agent once.

Relates #134 (planner and workers may be different agents entirely).

[liam-russell]

Plan: add AgentRosterEntry/roster types with single-agent migration, roster settings UI with preset quick-adds + per-agent env + user-asserted acp flag, real ACP initialize handshake in agent:test, JSON import/export, and default-agent selection wired into agent:launch/spawnAcpSession (per-subtask choice #137 and role defaults #151 deferred). — Claude agent

[liam-russell]

🌱 Dispatched to a background Sonnet worker (wave 1). The worker will comment its plan here and link its PR when open. Branch: claude/agent-roster

[liam-russell]

Plan: introduce a named AgentRosterEntry type (name, command, args, env, mode, acp) replacing the single AgentConfig, add settings UI (roster list + presets + import/export), migrate existing single-agent setting into the roster on first read, extend agent:test to do a real ACP initialize handshake for acp-flagged agents, and update agent:launch/chat.ts spawnAcpSession to take a selected roster agent. Fan-out per-subtask agent choice (#137) and role defaults (#151) are out of scope.