Fleet Studio: owner-routed open of a peer session + retire orphaned dropdown helpers (B2a, from #1525)

[jeonghun-jj-lee]

Important

Problem — After B1 (#1536), a peer's sessions appear in the titlebar Sessions dropdown, badged and searchable — but clicking one is a dead end: the openSession guard intentionally short-circuits to a "lives on " toast instead of navigating, because opening a remote session was never wired. JJ can see his Mac Studio's "Free port 4096" session but cannot open it.
Approach — Wire the dropdown's openSession for a remote (peer-owned) row to route through the owner via the existing multiplex/attach path, landing in the normal session view, instead of the B1 guard toast. This is a read path — it opens and views a peer session; it adds no mutation surface. Alongside, retire the orphaned filterSessionsByMachine / deriveBadge helpers that live only in the test file into a real shared module the component and the test both import.
Scope — in: owner-routed open of a peer session (AC4 of #1525); promote orphaned dropdown helpers into a real module (AC6 of #1525). out: remote prompt / archive / delete (AC5 — the remote-write surface); the evaluateRemoteWriteGate confirm wiring and its UX (deferred to B2b, pending design sign-off).
Assumptions — the B1 projection fetch/merge/badge plumbing in SessionChatsDropdown is merged and correct (PR #1536, merged 2026-09-24 13:32Z); the owner-routing seam B1 already guards against (SessionMultiplexProxy.resolve routes reads by owner) is the open path; opening is authorized at the API boundary by the reader token already on file (proven live: GET /session → 200 through the forward).

Acceptance Criteria

• Opening a peer (remote, is_local === false) session from the dropdown routes to the owner and lands in the session view — no bogus local navigate, no 404, no B1 guard toast.
• Opening a local session from the dropdown is byte-unchanged from today (no regression).
• When the projection is unavailable (404/empty), the dropdown behaves exactly as B1's degrade-to-local (peer rows simply absent; no throw).
• The dropdown's machine helpers (filterSessionsByMachine / deriveBadge, and any B1 merge helper still duplicated) live in ONE real module imported by BOTH the component and its test — no test-only re-implementation remains.
• No remote-write path is added: prompt / archive / delete of a peer row remain disabled or absent exactly as B1 left them.

Testing Decisions

Extend the existing session-component unit suite that B1 added (session-fleet-peers module tests + the SessionChatsDropdown component tests) — reuse-first. Add cases for: remote-row open dispatches the owner-routed navigate; local-row open unchanged; the promoted helper module is the single source (delete the orphaned copies in session-chats-dropdown.test.ts and import the real module — AC6 is proven by the test file no longer defining them). No new suite; this is the same surface B1 touched.

Key Decisions

• Open = read, and the multiplex already routes reads by owner; B2a reuses that seam rather than inventing a new transport. The B1 guard becomes a real navigate for remote rows.
• AC6 (helper retirement) rides with B2a because it is pure cleanup of the same dropdown subtree and touches no mutation surface — it does not belong in the design-gated B2b.
• The remote-write confirm gate (evaluateRemoteWriteGate) is explicitly not wired here; B2a must not create a live caller for it.

Constraints & Invariants

• Flag-OFF / no-projection behavior is byte-identical to B1's degrade-to-local — the peer-merge already ships behind the fleet posture; B2a adds no new default-on behavior.
• No new mutation surface. A reviewer must be able to confirm B2a cannot delete, archive, or prompt a remote session.
• Local-session behavior unchanged (open, badge, search all as B1 left them).

Prior Art

• The B1 pure module and the dropdown wiring it landed (session-fleet-peers + SessionChatsDropdown in the session-header subtree) — read these first; B2a extends them.
• The owner-routed multiplex seam (SessionMultiplexProxy.resolve, method-agnostic owner routing) — the open path.
• The orphaned helpers in session-chats-dropdown.test.ts — the AC6 target.

Source

• Part of Complete and base-enable dormant Fleet Studio slices #1475 · carved from Fleet Studio: titlebar Sessions dropdown must merge peer sessions from the fleet projection (badge + owner-routed control) #1525 (which retains AC5 as B2b) · Blocked by nothing (base has Fleet Studio #1524: base peer-observation routes mount without hub-activation #1535 + Fleet Studio #1525 (B1, read-only): peer sessions appear in the titlebar dropdown #1536 merged).

Notes

Carved from #1525 per the B2 split decision (2026-09-24): #1525 keeps AC5 (remote prompt/archive/delete behind the confirm gate) as the design-gated B2b; this issue is the safe two-thirds (AC4 open + AC6 helper retirement). B2b awaits a brainstorming pass on the remote-write safety model before it gets its own issue.