docs(agc): align design docs with shipped v0.18.1

[thanhleviet]

Summary

Refresh the hand-written AGC design ADRs under docs/design/ so their status lines and superseded notices match the shipped v0.18.1 reality. Follow-up to the AGC balanced-migration and hosted-index work (#35, #37) and the OSF link registry (#39).

The generated CLI reference under docs/reference/cli/ self-heals through make docs, but the design ADRs are hand-written and had drifted: several notices were still stamped v0.18.0-beta.3 and described the old three-node-crawl default, even though the feature graduated to stable in v0.18.0 and now defaults to a hosted OSF index TSV as of v0.18.1.

Changes

• Refresh the superseded notices across six AGC design docs to cite v0.18.1 and point at the current design (agc-osf-balanced-migration.md).
• Reconcile the accession-search ADR (sections 4-5) with the as-built code signatures.
• Add an As-built (v0.18.1) callout to the balanced-migration ADR (the current design) recording the one deviation from the plan: the combined index shipped on OSF node h7wzy, the primary ATB project node that also hosts the parquet metadata and sketchlib, not z7q5y as the plan text stated. The shipped AGCIndexURL resolves to h7wzy, folder metadata_balanced_batches/atb_agc_files.tsv, md5 8aea3d79da3e2a0af10c9904d0c3a10f, 1,268 batches.

Scope and verification

• Docs only: 7 files, +97 / -17, all under docs/design/. No code, generated reference, or config touched.
• mkdocs build --strict passes clean with no dangling links; the historical ADR bodies are preserved and only dated notices were appended.