chore(peering): retire discover-peers execution path (PLT-452 PR-C)

[bdchatham]

What

The final step of the discover-peers retirement (PLT-452). Bumps the seictl pin to v0.0.57 — the first release without the sidecar discover-peers task handler (removed in seictl#201) — and drops the two controller-side references the bump leaves dangling:

• internal/task/task.go — the discover-peers execution registration (sidecar.TaskTypeDiscoverPeers: sidecarTask[sidecar.DiscoverPeersTask]). Deliberately kept through PR-A/PR-B so any in-flight persisted plan could still drive its task to completion against an older sidecar; removable now that the controller stopped emitting them (refactor(nodetask): retire the imperative DiscoverPeers SeiNodeTask (PLT-452, PR-A) #398, live) and those tasks have drained.
• internal/planner/planner.go — the TaskDiscoverPeers alias over the now-removed sidecar task-type constant.

Both reference symbols that no longer exist in seictl v0.0.57, so they go with the bump.

Test

• internal/planner/archive_test.go — drops the TestArchivePlanner_WithPeers "must not contain discover-peers" assertion (and the types slice that fed it). It's structurally unreachable now: no enum value, no registry entry, no planner can emit that task type. The test keeps its meaningful assertion — peers reach config via the persistent_peers override — and the comment is refocused accordingly.

Sequencing

This is PR-C of the three-PR cross-repo retirement:

• PR-A (refactor(nodetask): retire the imperative DiscoverPeers SeiNodeTask (PLT-452, PR-A) #398, merged) — controller stops emitting discover-peers tasks.
• PR-B (seictl#201, merged → v0.0.57) — sidecar drops the handler + client API.
• PR-C (this) — controller bumps to v0.0.57 and drops the kept execution registration + alias.

Peering is now wholly controller-owned via the config-apply persistent_peers override; no discover-peers task type exists anywhere in the stack.

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Dependency bump plus deletion of dead task-type wiring; peering behavior is unchanged and already flows through config-apply overrides.

Overview
Completes discover-peers retirement by pinning seictl v0.0.57 (sidecar handler removed) and removing the last controller hooks that referenced that task type.

internal/task/task.go drops the discover-peers entry from the task execution registry so persisted plans can no longer deserialize or run that sidecar task. internal/planner/planner.go removes the TaskDiscoverPeers alias constant.

internal/planner/archive_test.go updates the peers test to only assert that resolved peers land in the config-apply persistent_peers override; the obsolete “plan must not contain discover-peers” check is removed because the type no longer exists anywhere in the stack.

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