ADR 0118: Preset config fills never overwrite a present value

Amendments.

• Vocabulary: current spellings are standards (formerly ratchets), [jobs] / [jobs.<name>] (formerly [capabilities] / [checks.<name>]), and known/custom job (formerly gate capability / custom check); the decision and reasoning are unchanged.
• ADR 0365 — preset withdrawal: the command and its config-fill result fields were removed before the first public tag. The fill-if-absent implementation remains only where a live caller owns it.

Status: superseded by ADR 0365

A preset is a file overlay plus config fills (ADR 0007/0018). The file half has always been create-or-skip: a present file is the user's and is never overwritten. The config half was not — applyConfigDoc wrote every fill through the comment-preserving editor's replace-existing path, so a preset whose preset.json carried capabilities.test silently replaced a user-authored test command in discern.toml. Nothing disclosed which keys would change: dry-run reported only a boolean, the confirm review one summary line. A user who had tuned their gate (cargo test --workspace --all-features) and applied a preset ended up with the weaker preset command and a silently narrower quality gate — with no per-key trail to notice it by.

The replace-existing path was never needed by the legitimate flows: setup --config applies fills to a freshly generated template (whose known-job entries ship commented out), and a fresh overlay writes into empty slots. The path only ever fired against values a user had authored.

Preset config fills are fill-if-absent, never overwrite — the same rule as the file half. A fill whose target already carries a real value (a set key, or a present [jobs.*] / [scopes.*] / [standards.*] table) is skipped; a commented-out template hint does not count as a value. applyConfigDoc gains a skipExisting mode and returns a per-path report (filled / skipped), which preset surfaces everywhere the user decides or reviews: the dry-run (JSON arrays config_fills_applied / config_fills_skipped and per-key human lines), the confirm review, and the apply result.

Explicit nos:

• setup --config keeps the default overwrite mode — it fills a just-generated template where the document is the user's declared intent, and nothing user-authored exists yet.
• No clean-tree guard was added to preset (unlike upgrade, ADR 0014): with overwrite gone, the verb no longer destroys anything uncommitted — both halves only add.
• No --force overwrite flag: a user who wants the preset value for a key they already set changes it with discern config set, keeping one mental model (present value = yours, always).

• Applying a preset is now idempotent and safe over a tuned config: re-applying fills nothing and leaves discern.toml byte-identical.
• A preset can no longer "upgrade" a project's existing commands even when the author intends it to; that is deliberate — config changes to values the user set are theirs to make, with the skipped keys disclosed so they know which to consider.
• The preset result schema grew two disclosure arrays; config_fills stays a boolean meaning "discern.toml was (or would be) written".
• A class-level guard (tests/config_doc_test.ts) derives the fill sections from the config-doc schema, so a new fill section cannot ship without enrolling in the never-overwrite test.