ListSessions returns IDs only — add cwd, currentModelId, inflight for operator visibility

[wjueyao]

Summary

AgentRunListSessionsResult currently returns only session IDs:

type SessionListResult struct {
    SessionIDs []string `json:"sessionIds"`
}

For operators inspecting multi-session pools, the missing fields (cwd, current model, in-flight prompt count, optionally created-at) require a follow-up RPC per session, or aren't available at all. The runtime already tracks (id, cwd, models, inflight) on sessionState; surfacing it through the wire layer is mostly plumbing.

Proposed shape

type SessionInfo struct {
    ID             string `json:"id"`
    Cwd            string `json:"cwd"`
    CurrentModelID string `json:"currentModelId,omitempty"`
    Inflight       int    `json:"inflight"`
}

type SessionListResult struct {
    Sessions []SessionInfo `json:"sessions"`
}

(Old SessionIDs []string shape goes away — pure forward-compat add since list-sessions is new in this multi-session work and hasn't shipped to a public release yet. If we wait, the field becomes load-bearing and harder to evolve.)

Touched

• pkg/agentrun/api/types.go: SessionListResult + new SessionInfo
• pkg/agentrun/runtime/acp/runtime.go: new Manager.Sessions() []SessionInfo (or extend SessionIDs() — name TBD)
• pkg/agentrun/server/service.go: ListSessions handler maps runtime → wire
• pkg/ari/api/types.go: ari mirror types
• pkg/ari/server/server.go: ari adapter passthrough
• pkg/ari/client/client.go: client decoding
• cmd/massctl/commands/agentrun/list_sessions.go: render table instead of one-line-per-id

Estimate

~80 LOC + 2-3 tests, ~45-60 min.

Why deferred from #2

Surfaced in PR #2's second review pass. #2 already focuses on the multi-session feature primitive; this is a feature expansion on top — IDs-only is functional, just not maximally useful. Keeping it as a follow-up keeps PR #2's scope clear and lets each piece be reverted independently.