feat(metadata|docs): reusable tag groups with $group: (#5037)

* feat(metadata): reusable tag groups with `$group:`

A `$group:<group-name>` marker in `tags` is replaced in place with the tags of
`groups/<group-name>.json`: a small set of tags for what a variant suffix
means, such as `-minus` → remove, delete.

- Read groups in `readGroups` and resolve markers next to `$extends:`,
  failing on unknown groups and on `$group:` outside `tags`
- Add `group.schema.json` and lint, spell-check and format `groups/*.json`
- `pnpm checkIcons` also fails on literals a group already provides, and
  warns about icons that list a group's tags by hand and unused groups
- Add the `minus`, `plus` and `x` groups and use them in
  `alarm-clock-minus`, `file-minus`, `file-plus` and `file-x`

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Fix group checker

* Apply batched suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* feat(metadata): reusable tag groups with `$group:`

A `$group:<group-name>` marker in `tags` is replaced in place with the tags of
`groups/<group-name>.json`: a small set of tags for what a variant suffix
means, such as `-minus` → remove, delete.

- Read groups in `readGroups` and resolve markers next to `$extends:`,
  failing on unknown groups and on `$group:` outside `tags`
- Add `group.schema.json` and lint, spell-check and format `groups/*.json`
- `pnpm checkIcons` also fails on literals a group already provides, and
  warns about icons that list a group's tags by hand and unused groups
- Add the `minus`, `plus` and `x` groups and use them in
  `alarm-clock-minus`, `file-minus`, `file-plus` and `file-x`

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Fix group checker

* Apply batched suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* feat(metadata): reusable tag groups with `$group:`

A `$group:<group-name>` marker in `tags` is replaced in place with the tags of
`groups/<group-name>.json`: a small set of tags for what a variant suffix
means, such as `-minus` → remove, delete.

- Read groups in `readGroups` and resolve markers next to `$extends:`,
  failing on unknown groups and on `$group:` outside `tags`
- Add `group.schema.json` and lint, spell-check and format `groups/*.json`
- `pnpm checkIcons` also fails on literals a group already provides, and
  warns about icons that list a group's tags by hand and unused groups
- Add the `minus`, `plus` and `x` groups and use them in
  `alarm-clock-minus`, `file-minus`, `file-plus` and `file-x`

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Fix group checker

* Apply batched suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

* feat(metadata): reusable tag groups with `$group:`

A `$group:<group-name>` marker in `tags` is replaced in place with the tags of
`groups/<group-name>.json`: a small set of tags for what a variant suffix
means, such as `-minus` → remove, delete.

- Read groups in `readGroups` and resolve markers next to `$extends:`,
  failing on unknown groups and on `$group:` outside `tags`
- Add `group.schema.json` and lint, spell-check and format `groups/*.json`
- `pnpm checkIcons` also fails on literals a group already provides, and
  warns about icons that list a group's tags by hand and unused groups
- Add the `minus`, `plus` and `x` groups and use them in
  `alarm-clock-minus`, `file-minus`, `file-plus` and `file-x`

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Fix group checker

* Apply batched suggestions from code review

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

.github/instructions/groups.instructions.md

@@ -0,0 +1,29 @@
+---
+applyTo: "groups/*.json"
+---
+# Groups
+
+A group is a small, reusable set of tags for **one icon variant** — the modifier suffix in an icon name, such as `-plus`, `-check` or `-off`. Icons reference a group with a `"$group:<group-name>"` marker inside their `tags` array, and the marker is replaced at build time with the group's tags.
+
+The file name without `.json` is the group name: the variant suffix it belongs to (`plus` for `file-plus`, `question-mark` for `file-question-mark`), in lowercase kebab-case. Each file has a `$schema`, a one-line `description` saying which meaning the group covers, and a non-empty `tags` array.
+
+```json
+{
+  "$schema": "../group.schema.json",
+  "description": "Action: removing or deleting something",
+  "tags": ["remove", "delete"]
+}
+```
+
+## Rules
+
+- **One group per variant.** Name the group after the variant suffix (`plus`, `minus`, `check`, `cog`), so `file-plus` uses `$group:plus` and `alarm-clock-minus` uses `$group:minus`.
+- **Different suffixes with the same meaning share one group.** Do not create a second group for a synonym: `-warning` and `-exclamation-point` use `$group:alert`. When a meaning is spread over many suffixes, name the group after the meaning instead: every currency suffix (`-euro`, `-dollar-sign`, `-japanese-yen`…) uses `$group:currency`.
+- **Tags describe what the variant means, not its glyph.** `plus` holds `add`, `new`, `create`; `cog` holds `settings`, `gear`. Do not put the suffix itself (`plus`, `cog`, `lock`) in a group: tags must not repeat the icon name.
+- **Keep groups small and generic.** Only tags that hold for *every* icon that uses the group. If a tag is true for just some of them, it belongs on those icons as a literal.
+- **Not every icon with the suffix uses the group.** `-off` means "disabled" on `wifi-off` but "allergy free" on `wheat-off`, and `-x` means "delete" on `file-x` but "mute" on `volume-x`. Only icons where the variant carries the group's meaning use the marker; the others keep literal tags.
+- **Do not add a group before it is used.** `pnpm checkIcons` warns about groups no icon references.
+- **Groups do not nest.** A group's `tags` are literal tags only — no `$extends:` or `$group:` markers.
+- Tags are lowercase, and follow the same conventions as icon tags.
+
+Run `pnpm run lint:json:groups` to validate the files against the schema, and `pnpm checkIcons` to check how icons use them.