feat(site): an older label file keeps building; abcd adds only the labels it lacks (TG1)

[REPPL]

An older website label file keeps building: when a repository's site-src/ui.json was written before a label existed, abcd site setup and abcd site build add only the labels it lacks, with abcd's default words, and leave everything the project wrote exactly as it was.

This applies the product thinker's ruling TG1 of 2026-09-30, verbatim: "(b) ABCD ADDS THE MISSING LABELS: on the next site setup or site build, abcd adds only the missing required labels (with the default words); the project's own wording elsewhere in ui.json is never changed. No failure, no manual step; both intents stay impact: additive."

What changes

• addMissingLabels (internal/core/site/uiadd.go) adds each label the allowlist declares and the file does not carry. Existing values stay byte for byte, new members go at the end of their block in its own indentation, a whole missing block is added whole, and the file is written atomically keeping its mode. A symlinked or non-regular ui.json is refused as the site's other reads refuse it.
• A blank declared label is the project's own and is still refused by name. A file carrying a key abcd does not declare is left untouched and still refused by the closed allowlist.
• site build runs it before the labels are loaded; site setup runs it after the repository stage, reports the file written and names it in the commit step. Both print one stderr line per added label and carry added_labels and labels_file in the JSON result.
• The site gate's own render (abcd lint site on an empty output directory) does not complete the file, so the gate still writes only inside its output directory.

The record

• adr-2609301720596683 records TG1 and the one exception to "a file the repository owns once it exists is kept"; it refines adr-47 and leaves decision 2's closed allowlist untouched.
• The site brief chapter and commands/site.md state the behaviour; one DECISIONS.md line is appended; itd-2609212103568351 and itd-2609212103572513 keep impact: additive, each with an Audit Notes line citing TG1.

Assisted-by: Claude:claude-opus-5-5