burn-down(docs): relocate config-manager — docs/ fully evacuated (Design 05 Phase-C)

[bdchatham]

Phase-C (paired add: bdchatham-designs#22). Removes docs/config-manager/DESIGN.md (relocated to the designs repo). It was the last doc in docs/ — this fully evacuates docs/, so sei-config reaches the Design 05 end-state invariant (no docs/ folder). Inbound audit: 0 refs, clean removal. ⚠️ merge bdchatham-designs#22 first. Linear: PLT-497

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[cursor]

PR Summary

Low Risk
Documentation-only deletion with no runtime or library changes; merge order depends on the inbound designs-repo PR.

Overview
Removes docs/config-manager/DESIGN.md as part of Design 05 Phase-C burn-down; the design content lives in the paired bdchatham-designs PR instead of sei-config.

This was the last file under docs/, so the PR eliminates the entire docs/ tree in this repo and satisfies the “no docs/ folder” end-state for sei-config. No code or config behavior changes—documentation relocation only.

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