burn-down(docs): relocate 4 design artifacts to bdchatham-designs (Design 05 Phase-C)

[bdchatham]

Phase-C fan-out, seictl batch (Design 05 / PLT-497) — the burn-down half (paired add: bdchatham-designs#20).

Removes 4 design artifacts now relocated to the designs repo, moves the runbook to .agent/runbooks/, and repoints inbound cross-links:

doc	disposition
docs/design/cluster-cli.md	→ bdchatham-designs designs/cluster-cli/
docs/design/in-pod-governance-signing.md	→ designs/governance-signing/ (+ 2 keyring.md cross-links repointed)
docs/design/validation-substrate.md	→ designs/validation-substrate/ (corpus exemplar)
docs/bugbash/cluster.md	→ designs/bugbash-cluster/
docs/runbooks/seictl-smoke.md	→ .agent/runbooks/ (stays in-repo)
docs/design/nodedeployment-cli.md	stays — Sonar-gated (real ref in nodedeployment/cmd.go:5)

Audit (canonical origin/main 62b06dd, gating): in-pod-governance-signing had 2 documentation cross-links in sidecar/docs/keyring.md (repointed here); the other three were clean of any inbound refs.

docs/ is not fully evacuated — nodedeployment-cli.md remains until Sonar (PLT-493). This was prepared in an isolated git worktree off origin/main, leaving the active feature branch untouched.

⚠️ Merge order: merge bdchatham-designs#20 first, then this. · Linear: PLT-497

🤖 Generated with Claude Code

[cursor]

PR Summary

Low Risk
Documentation-only relocation with no runtime or CLI behavior changes; link rot is mitigated by merging the designs repo PR first.

Overview
Design 05 / PLT-497 burn-down: four large design documents are removed from seictl now that canonical copies live in bdchatham-designs (cluster-cli, in-pod-governance-signing, validation-substrate, and docs/bugbash/cluster.md).

sidecar/docs/keyring.md is updated so both references to the governance design point at the external repo instead of the deleted in-tree path.

The harbor seictl smoke runbook is relocated to .agent/runbooks/seictl-smoke.md (still in this repo). docs/design/nodedeployment-cli.md is intentionally left in place (Sonar / PLT-493).

Merge bdchatham-designs#20 before this PR so outbound links resolve.

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