Add README, install guide, AGENTS.md, and plugin manifest

- README.md: overview, quick start, available skills
- INSTALL.md: plugin install + manual symlink methods
- AGENTS.md + .claude/CLAUDE.md: dev context for Claude Code
- .claude-plugin/: plugin.json + marketplace.json for
  /install-plugin github:basecamp/skills

.claude-plugin/marketplace.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://anthropic.com/claude-code/marketplace.schema.json",
+  "name": "basecamp-skills",
+  "metadata": {
+    "description": "Basecamp skills for Claude Code",
+    "version": "0.1.0"
+  },
+  "owner": {
+    "name": "37signals",
+    "email": "support@37signals.com"
+  },
+  "plugins": [
+    {
+      "name": "basecamp-skills",
+      "description": "Basecamp skills for Claude Code — project management, todos, cards, messages, and more.",
+      "source": "./",
+      "category": "productivity"
+    }
+  ]
+}

.claude-plugin/plugin.json

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+{
+  "name": "basecamp-skills",
+  "version": "0.1.0",
+  "description": "Basecamp skills for Claude Code — project management, todos, cards, messages, and more.",
+  "author": {
+    "name": "37signals",
+    "email": "support@37signals.com"
+  },
+  "homepage": "https://github.com/basecamp/skills",
+  "repository": "https://github.com/basecamp/skills",
+  "license": "MIT"
+}

.claude/CLAUDE.md

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+@../AGENTS.md

AGENTS.md

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+# Basecamp Skills Repo
+
+Distribution repo for Basecamp skills for Claude Code. Skills are auto-synced
+from [basecamp/basecamp-cli](https://github.com/basecamp/basecamp-cli) on each
+release by `scripts/sync-skills.sh`.
+
+## Key constraints
+
+- **Do not edit skills here.** Edit in `basecamp-cli/skills/` and let the sync
+  publish them. Manual edits will be overwritten on the next release.
+- `.managed-skills` tracks which skill directories are owned by the sync script.
+  Don't edit it manually.
+- Non-skill files (README, AGENTS.md, INSTALL.md, .claude-plugin/) are safe to
+  edit directly — the sync only touches `skills/` and `.managed-skills`.
+
+## Repo structure
+
+```
+.claude-plugin/        # Plugin manifest for /install-plugin
+skills/
+  basecamp/SKILL.md    # Synced from basecamp-cli
+.managed-skills        # Auto-generated manifest
+```

INSTALL.md

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+# Installation
+
+## As a plugin (recommended)
+
+```bash
+/install-plugin github:basecamp/skills
+```
+
+This installs all skills and keeps them updated.
+
+## Manual installation
+
+### Personal (all projects)
+
+```bash
+git clone https://github.com/basecamp/skills ~/.basecamp-skills
+
+# Symlink individual skills
+ln -s ~/.basecamp-skills/skills/basecamp ~/.claude/skills/basecamp
+
+# Or all at once
+for skill in ~/.basecamp-skills/skills/*/; do
+  ln -s "$skill" ~/.claude/skills/"$(basename "$skill")"
+done
+```
+
+### Per-project
+
+```bash
+# From your project root
+ln -s ~/.basecamp-skills/skills/basecamp .claude/skills/basecamp
+```
+
+### Updating
+
+```bash
+cd ~/.basecamp-skills && git pull
+```
+
+Symlinks pick up changes immediately. If you copied instead of symlinking,
+re-copy after pulling.
+
+## Prerequisites
+
+The **basecamp** skill requires the [Basecamp CLI](https://github.com/basecamp/basecamp-cli):
+
+```bash
+brew install basecamp/tap/basecamp
+basecamp auth login
+```

README.md

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+# Basecamp Skills for Claude Code
+
+Skills that extend [Claude Code](https://docs.anthropic.com/en/docs/claude-code)
+with Basecamp project management capabilities — todos, cards, messages,
+schedule, campfire, and more.
+
+## Quick start
+
+```
+/install-plugin github:basecamp/skills
+```
+
+Or see [INSTALL.md](INSTALL.md) for manual installation.
+
+## Available skills
+
+| Skill | Description |
+|-------|-------------|
+| [basecamp](skills/basecamp/SKILL.md) | Full Basecamp CLI integration — 130 endpoints across todos, cards, messages, files, schedule, check-ins, timeline, recordings, templates, webhooks, subscriptions, lineup, and campfire. |
+
+## Requires
+
+- [Basecamp CLI](https://github.com/basecamp/basecamp-cli) (`brew install basecamp/tap/basecamp`)
+- An authenticated Basecamp account (`basecamp auth login`)
+
+## About
+
+These skills are automatically published from
+[basecamp/basecamp-cli](https://github.com/basecamp/basecamp-cli) on each
+release. To contribute, open issues or PRs there.