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>

extras/git-guardrails-claude-code/scripts/block-dangerous-git.sh

@@ -0,0 +1,25 @@
+#!/bin/bash
+
+INPUT=$(cat)
+COMMAND=$(echo "$INPUT" | jq -r '.tool_input.command')
+
+DANGEROUS_PATTERNS=(
+  "git push"
+  "git reset --hard"
+  "git clean -fd"
+  "git clean -f"
+  "git branch -D"
+  "git checkout \."
+  "git restore \."
+  "push --force"
+  "reset --hard"
+)
+
+for pattern in "${DANGEROUS_PATTERNS[@]}"; do
+  if echo "$COMMAND" | grep -qE "$pattern"; then
+    echo "BLOCKED: '$COMMAND' matches dangerous pattern '$pattern'. The user has prevented you from doing this." >&2
+    exit 2
+  fi
+done
+
+exit 0