v3.5: config editing via webui/Studio — schema API from config_key_docs + all writes through yaml_edit

[kengio]

Maintainer direction (2026-07-10)

onebrain.yml is the single source of truth — zero tolerance for wrong writes. v3.5+ will let users edit config directly from the webui/Studio; those edits MUST land on the right key in the right section, always.

Architecture (locked direction)

• One schema, one editor, every surface. config_key_docs() (onebrain-fs) already drives the init template, doctor --fix comment backfill, and section layout/restructure (v3.4.8 feat(config): self-documenting onebrain.yml template + doctor validate/reset-to-default #199/v3.4.8 T2b: config template section layout (banners, recap/schedule docs, stats last) + doctor --fix full restructure #203/feat(config): section layout for onebrain.yml + doctor --fix restructure (#203) #206, guarded by a completeness test binding it to the VaultConfig serde structs). yaml_edit is the only sanctioned writer (comment-preserving; v3.4.8 T2c (final track): migrate the 4 remaining comment-dropping onebrain.yml writers onto yaml_edit (fix_vault_yml_keys · fix_legacy_qmd_collection · persist_search_key · remove_search_key) #200 migrates the last four legacy serde writers onto it).
• New: config-schema API — daemon endpoint (e.g. GET /api/config/schema) exposing every key's path, section, doc string, default, and validation range (same sources the doctor config-values check uses). The webui/Studio settings form renders FROM this schema — no hardcoded key lists in the frontend.
• New: config-write API — a daemon endpoint applying a single key edit via yaml_edit::set_value (+ validation-before-write from the same rules), so a webui edit is byte-preserving and lands in the correct section exactly like doctor --fix would.

Prerequisites

• v3.4.8 T2c (final track): migrate the 4 remaining comment-dropping onebrain.yml writers onto yaml_edit (fix_vault_yml_keys · fix_legacy_qmd_collection · persist_search_key · remove_search_key) #200 (v3.4.8 T2c) — all writers on yaml_edit (no bypass path left)
• Completeness guard test (feat(config): section layout for onebrain.yml + doctor --fix restructure (#203) #206) — schema can never lag the structs

Non-goals here

Frontend implementation details — this issue covers the CLI/daemon contract.