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>

drafts/claude-handoff/SKILL.md

@@ -0,0 +1,18 @@
+---
+name: claude-handoff
+description: Hand the current conversation off to a fresh background agent that picks up the work immediately.
+argument-hint: "What will the next session be used for?"
+disable-model-invocation: true
+---
+
+Write a handoff summary of the current conversation so a fresh agent can continue the work. Instead of saving it, launch a background agent seeded with the summary as its prompt: `claude --bg --name "<descriptive name>" "<handoff summary>"`. It starts in the current working directory and returns immediately; the user manages it with `claude agents`.
+
+Always pass `-n`/`--name` with a descriptive name (e.g. `--name "Fix login bug"`) — it sets the display name shown in the job list, session picker, and terminal title.
+
+Include a "suggested skills" section in the summary, which suggests skills that the agent should invoke.
+
+Do not duplicate content already captured in other artifacts (specs, plans, ADRs, issues, commits, diffs). Reference them by path or URL instead.
+
+Redact any sensitive information, such as API keys, passwords, or personally identifiable information — the summary becomes the agent's prompt.
+
+If the user passed arguments, treat them as a description of what the next session will focus on and tailor the summary accordingly.

drafts/claude-handoff/agents/openai.yaml

@@ -0,0 +1,5 @@
+interface:
+  display_name: "Claude Handoff"
+  short_description: "Hand off to a background agent"
+policy:
+  allow_implicit_invocation: false