Fleet Studio #1525 (B1, read-only): peer sessions appear in the titlebar dropdown

[jeonghun-jj-lee]

Part of #1525 (B1 — read-only half). Base: feature/free-tier-fleet. Depends functionally on #1524 (PR #1535).

What & why

The titlebar Sessions dropdown never showed peer sessions because SessionChatsDropdown (session-header.tsx) builds its list purely from local project stores + the local archived list — it never fetched /amicode/fleet/sessions and never read amicode_owner. (The machine-filter helpers lived only in session-chats-dropdown.test.ts, wired to nothing.)

B1 makes peer sessions appear — badged, searchable, de-duped — with no destructive control. Owner-routed open and remote prompt/archive/delete (+ the evaluateRemoteWriteGate confirm) are B2, left in #1525 for design sign-off.

Change

• New pure module session-fleet-peers.ts (unit-tested, 9 cases):
  • peerSessionsFromProjection — tolerant reader (404/error/malformed → []), keeps only is_local:false entries, coerces to the SDK Session shape + owner overlay.
  • mergePeerSessions — dedupe by id (local wins), sort by last activity.
  • deriveSessionBadge / isRemotePeerSession.
• SessionChatsDropdown: fetch /amicode/fleet/sessions while the flyout is open (tolerant createResource), merge peers into the active list, render the owner badge (monitor icon, matching the provenance header), and
  • guard openSession — a remote row does not do a broken local navigate; it surfaces "lives on <machine>".
  • hide the local archive action on remote rows.

Safety / degradation

• No projection route (404) or no fleet → the dropdown is the local list, exactly as before. No throw, no blank.
• Read-only: adds no control authority over a peer's sessions.

Gates

• Session-component unit tests 62/62 (incl. the new module 9/9 + the existing dropdown/provenance tests — no regression)
• Overlay typecheck clean across all 7 packages (app/ui/session-ui/core/schema/sdk/llm)
• Full app unit suite: 12 failures, all pre-existing happydom-environment flakes in unrelated modules (observe-element-offset, prompt-input editor, session relay routes, icon theme bridge, postAmicode bridge). Proven unrelated: nothing outside session-header.tsx imports the changed subtree.

ACs (of #1525)

• AC1 remote peer sessions appear, badged ✓
• AC2 projection 404/empty → local list unchanged ✓
• AC3 search matches merged peer sessions ✓ (existing search runs over the merged list)
• AC4 owner-routed open · AC5 control + confirm gate · AC6 retire orphaned test helpers → B2 (deferred, need design sign-off).

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