refactor: flatten skills/ and adopt Agent Plugins 1.0

The Agent Plugins standard reads only the immediate children of
skills/ and cannot be pointed elsewhere, so a bucketed tree is
invisible to every conformant client.

Rather than generate a flat copy of the repo into a package
directory — a builder, a validator, an allowlist, a drift CI job,
and every skill committed twice — remove the buckets.

- skills/ is flat and holds exactly the promoted 25
- in-progress/ -> drafts/, misc/ -> extras/, deprecated/ deleted
- new root plugin.json (Agent Plugins 1.0)
- .claude-plugin/plugin.json loses its 25-entry skills array
- docs/ keeps its category folders; it is now the only place a
  skill's category lives

Reasoning in .agents/adr/0003.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/link-skills.sh

@@ -5,7 +5,8 @@ set -euo pipefail
 # It is not a supported installer. Modifications to it — or requests for
 # modifications — will not be approved.
 #
-# Links all skills in the repository into the local skill directories used by
+# Links every skill in the repository — promoted (`skills/`) as well as
+# unpromoted (`drafts/`, `extras/`) — into the local skill directories used by
 # each agent harness:
 #   - ~/.claude/skills  — Claude Code
 #   - ~/.agents/skills  — Codex and other Agent Skills-compatible harnesses
@@ -22,7 +23,7 @@ while IFS= read -r -d '' skill_md; do
   src="$(dirname "$skill_md")"
   names+=("$(basename "$src")")
   srcs+=("$src")
-done < <(find "$REPO/skills" -name SKILL.md -not -path '*/node_modules/*' -not -path '*/deprecated/*' -print0)
+done < <(find "$REPO/skills" "$REPO/drafts" "$REPO/extras" -maxdepth 2 -name SKILL.md -not -path '*/node_modules/*' -print0)
 
 for DEST in "${DESTS[@]}"; do
   # If $DEST is a symlink that resolves into this repo, we'd end up writing the