feat: side-by-side compare view across multiple instances

[BenSheridanEdwards]

Why

Multi-instance support landed in #8 — but the interface is still "pick one
active instance at a time." For users running a fleet of self-hosted Honchos
(e.g. one per agent), the question that motivates running them is "how do
my agents' representations of the same person differ?" — which can't be
answered without flipping between instances and reconciling by memory.

This PR adds a /compare route that renders N columns, one per pinned
instance, so peer representations can be inspected side-by-side without
disturbing the active instance.

What

API layer

• createScopedClient(instance) — openapi-fetch client bound to a specific
  instance rather than the active one. Sits next to the existing
  client.current pattern (no change to any existing query hook).
• compareQueries.ts — scoped versions of useWorkspaces, usePeers,
  usePeerRepresentation, usePeerCard. Each accepts an Instance and
  scopes its TanStack Query key by instance.id so caches don't collide
  across columns.

UI

• /compare route (top-level), search params: instances=<comma-ids> and
  peer=<target-name> for shareable/deep-linkable views.
• CompareView — header with target-peer input and "Add instance" dropdown
  (only shows instances not already pinned), plus the columns grid.
• CompareColumn — workspace selector, peer selector, peer representation
  list (top conclusions), and peer card via the existing PeerCardViewer.
  Auto-selects first workspace; prefers the target peer name if it exists
  in that instance, else first peer.
• Sidebar gets a Compare nav entry between Workspaces and Settings, using
  the Columns2 icon.

Tests

• scopedClient.test.ts — verifies requests target the instance's baseUrl
  and attach the instance's Bearer token.
• compare.test.tsx — verifies the empty-state prompt renders when no
  instances are selected.

Out of scope (potential follow-ups)

• Diff highlighting between columns (semantic diff of conclusion sets)
• Synchronized scrolling between columns
• "All peers" view (compare conclusion counts, last-dream times, etc. as a
  fleet-overview dashboard)

Manual verification

Verified against a local fleet of 5 Honcho stacks (one per agent, ports
8001–8005). Each column's queries correctly route to its own instance's
URL (confirmed via PerformanceResourceTiming). The visual layout renders
cleanly with 5 columns side-by-side.

Note: live data fetch requires the Honcho daemons to allow the dev origin
in their CORS config. The existing client.current health check hits
the same CORS path — this PR doesn't change that surface.

Conventions

• Conventional commit (feat:)
• TypeScript strict, no any
• CSS variables for theme colors (no Tailwind color utilities)
• TanStack Router as never cast at navigate callsites
• pnpm typecheck, pnpm lint, and the new tests all pass locally