[GFI] Add a Zed editor adapter

Goal

Add a zed adapter so Coco installs into the Zed editor's AI/agent configuration.

Files involved

• adapters/zed/manifest.json (new)
• adapters/zed/install.sh (new) — wire skills/commands/rules into Zed's config (or emit an AGENTS.md if Zed consumes one)
• adapters/zed/README.md (new)
• install.sh — optional detect_adapter() heuristic for Zed

Acceptance criteria

• bash install.sh --adapter zed installs without error into Zed's config location
• manifest.json validates; README documents the target paths and limitations
• CI passes (frontmatter linter, manifest validator, install script syntax check)
• README System Compatibility table gains a Zed row

Hints

• Start from adapters/generic (AGENTS.md producer) or adapters/cursor.
• Adding an adapter: CONTRIBUTING.md

Start with adapters/generic, adapters/cursor, and CONTRIBUTING.md to understand adapter conventions, then inspect install.sh and detect_adapter(). Verify Zed's configuration path before implementing the files listed in the issue. Done means --adapter zed installs without error, the manifest validates, the README documents paths and limitations, the compatibility table includes Zed, and CI checks pass.