feat: `vouch install-mcp <host>` — one-command adapter writer

[plind-junior]

Labels: enhancement, DX

What you're trying to do

Adoption today is "follow the README, write .mcp.json by hand, copy CLAUDE.md, hope you got it right." Each host (Claude Code, Cursor, Windsurf, Zed, Cline, JetBrains, Claude Desktop) has a slightly different config-file path. This is repetitive busywork that scales poorly.

The Claude Code adapter plan (docs/superpowers/plans/2026-05-27-claude-code-adapter.md) covers the writer for one host — generalize it.

Suggested shape

vouch install-mcp claude-code        # default
vouch install-mcp cursor --tier T2   # stop at CLAUDE.md
vouch install-mcp windsurf
vouch install-mcp claude-desktop
vouch install-mcp --list             # available hosts

• adapters/<host>/ directories ship the per-host templates (.mcp.json, CLAUDE.md fence, optional slash commands, optional settings).
• src/vouch/install_adapter.py (already specced) maps host → target paths and writes idempotently.
• A --target /abs/path flag lets users install into a project other than cwd.

Acceptance

• vouch install-mcp --list enumerates ≥6 hosts.
• Each writer drops files into the correct per-host path; existing files are left alone (fenced append for CLAUDE.md only).
• A test pack runs each host's writer against a tmp dir and asserts the expected files appear.

Out of scope

• Auto-detecting which host is running (do that explicitly).
• Uninstall (manually delete or revert with git).

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