docs(specs): remove the reverted spec 013

[bdchatham]

Summary

Removes docs/specs/013-nodeconfig-switch/spec.md and its row in docs/specs/README.md.

Spec 013 described the temporary in-place switch of a running SeiNode to spec.nodeConfig (#608). #610 reverted the code after the arctic-1 migration (PLT-1410) and kept the document, marked Reverted. The document now goes as well.

• No code changes. The spec has no code or test left to describe.
• No broken links. Nothing else in this repository links to the spec file. The runbook nodeconfig-seinode-operations.md names spec 013 by linking to feat(seinode): let a running node switch to nodeConfig (spec 013, temporary) #608, which keeps the full design for a later move.

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Documentation cleanup only; no code, APIs, or operational behavior changes.

Overview
Removes the reverted feature spec 013 (in-place nodeConfig switch for arctic-1) from the docs tree now that the behavior was reverted in #610 and no implementation remains.

Deletes docs/specs/013-nodeconfig-switch/spec.md and drops its row from the specs index in docs/specs/README.md. Documentation-only; no runtime or controller changes.

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[seidroid]

Settled by the scouts (codex) without a review turn: every one of them read the diff, found nothing, and reported it inert -- comments, documentation or whitespace only, with no executable surface changed.

_{seidroid review · decision approve · settled by the scouts codex · no review turn ran}

Findings: 0 blocking | 0 non-blocking | 0 posted inline