fix(fleet): sidebar client-side peer visibility (#1363)

[jeonghun-jj-lee]

Closes #1363. Fixes the reported bug: on a client machine the sidebar fleet section shows only itself; the canonical server never appears (on the server, both do).

Root cause

The sidebar's readRoster seam unconditionally read the machine-local roster cache — empty on a client, because the roster is host-owned (written only on the server via POST /amicode/roster). The client-side proxy read was an explicit #1321 follow-up. Compounding it, the server is never a roster row — it appears only via the machine's own self-row synthesis, so a client had nothing to render for it.

Changes (sidebar-only)

• readRoster is role-aware. SERVER/standalone read the local cache synchronously (unchanged); a CLIENT proxy-reads the host's authoritative roster over the existing GET /amicode/roster (forwarded by the local service). Any failure — no live endpoint, network error, non-200, malformed body — is the honest reachable:false state, never a fabricated or stale list. The seam returns a sync | Promise union, so every existing synchronous injection is untouched (no test churn).
• Canonical-server synthesis. buildFleetSectionModel synthesizes the server node on a client (from fleet.json canonical), mirroring the existing self-row synthesis; shown as down when the host is unreachable, and never doubled (self-row / existing roster row de-dupe).
• Live wiring. extension.ts passes a lazy service-endpoint getter (the service boots after sidebar construction → null until up ⇒ honest degrade).

Scope / non-goals

• Sidebar-only. The enroll verb, roster JSON schema, and never-fork guard are byte-unchanged (empty diff vs base — the additive invariant).
• The roster-row enrichment (friendly name + device_type — why the server currently renders the laptop as JJs-MacBook-Pro.local / client vs the laptop's own JJ's Macbook Pro / laptop) touches the schema + enroll verb and is deferred to ADR 0027 (Peer fleet studios + compute federation (ADR 0027) #1340 / PR Peer fleet studios (ADR 0027, Horizon 1) — #1341–#1346 #1353), which owns the roster schema.

Testing

• sidebar_fleet_section.test.ts: +4 pure-unit cases for the canonical-server synthesis (present / de-dupe / not-for-local / down-when-unreachable).
• sidebar_view.test.ts: +5 deps cases (client proxy-read with credential; client degrade on throw / no-endpoint; server reads local & never fetches; readCanonicalServer client-vs-server) +1 provider-level async end-to-end (client posts a fleet-status including the synthesized server node).
• Full extension suite green (3996 passed / 0 failed); tsc --noEmit clean.

Known follow-up (for the reviewer / ADR 0027)

If a server ever self-registers a roster row, its machine_id (hostname) may differ from the canonical host string used for synthesis — a de-dupe reconciliation that belongs with gap 3's roster-identity work.

HITL — do not merge on my account; human promotes. Base is feature/free-tier-fleet (not main).

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