fix(desktop): make local agents always use the workspace relay

[wpfleger96]

Local managed agents are an invariant-workspace-relay class, but several relay operations read each record's frozen per-record relay_url instead of the session's workspace relay. A record minted under an old build froze at ws://localhost:3000, so the boot reconcile loop hit a dead relay and printed a relay unreachable line on every just staging run.

Approach

Fix it at read time, with no on-disk migration. goose is a valid runtime (migrating it would corrupt working records), and rewriting relay_url would ping-pong the cross-worktree-symlinked managed-agents.json between worktrees with different relay overrides. Instead, a LOCAL agent always resolves the workspace relay at every point it touches a relay; a stale stored value becomes inert rather than load-bearing.

Changes

• relay::effective_agent_relay_url — a small pure helper centralizing the local-vs-remote decision: local agents use the workspace relay, remote (Provider) agents keep their per-record relay.
• Reconcile (reconcile_agent_profile) — both the profile read and the write-back route through the effective relay. ProfileReconcileData gains a backend_is_local flag set at both build sites (UI-start and boot).
• Spawn (spawn_agent_child) — BUZZ_RELAY_URL and the git-credential-helper URL both use the effective relay, so the child connects and authenticates on the workspace relay, not a frozen per-record value.
• Rename re-sync (update_managed_agent) — re-publishes a renamed local agent's profile to the workspace relay rather than a possibly-stale per-record relay.
• Create lockdown (create_managed_agent) — local agents always use the workspace relay and ignore any user-supplied relay_url, so the invariant holds at birth. The Provider arm is unchanged.
• Picker default (CreateAgentDialog.tsx) — new agents default to buzz-agent instead of goose at both the initial-state and reset sites.

Remote (Provider) agents are byte-for-byte unchanged across all four relay readers. Existing goose records are untouched — no command migration, no relay migration, nothing written to managed-agents.json to "correct" records.

The doc comments on reconcile_agent_profile and query_agent_profile that previously stated per-record targeting was intentional are updated to record the new local-vs-remote contract.

Note

desktop/scripts/check-file-sizes.mjs bumps the existing TEMP runtime.rs override (1940 to 1953); the file was at 1938 with two lines of slack, and the spawn fix is load-bearing. The file remains queued to be split.