Add Codex, Cursor, and OpenCode init options alongside Claude Code

[mlieberman85]

Summary

Today darnit install --client claude (see #270 for the Claude Code vs Claude Desktop nuance) is the only supported init flow. Coding-agent adoption is broader than Claude Code, and darnit's product path is skills + MCP tools invoked from a coding agent (per the memory note "CLI is not the product"). To meet users where they are, the install path should support:

• OpenAI Codex CLI (first-class alongside Claude Code)
• Cursor
• OpenCode (assumed to be sst/opencode or the equivalent -- please correct on this issue if it's a different tool)

Scope

• Extend darnit install to accept --client codex | cursor | opencode and register the darnit MCP server + skills into each client's expected config location.
• Per-client config paths, permission scopes, and MCP config formats differ; each client needs its own writer. Reuse the lesson from darnit install --client claude writes to ~/.claude/settings.json (Claude Desktop), not Claude Code's ~/.claude.json #270: settings-file locations vary even within a single vendor.
• Explicit --client flag; no auto-detection in v0. (Auto-detection can be a follow-up.)

Out of scope

• Building a shared abstraction over N clients (defer until third client is real code).
• Documentation improvements for each client's darnit setup story (separate issue if needed).
• Marketing / awareness work.

Related

• Packaging: distribute darnit as a Claude Code plugin (and other coding-agent platforms) #231 (Packaging: distribute darnit as a Claude Code plugin and other coding-agent platforms) -- adjacent packaging concern.
• darnit install --client claude writes to ~/.claude/settings.json (Claude Desktop), not Claude Code's ~/.claude.json #270 (Claude Desktop vs Claude Code settings.json divergence) -- template for per-client config surprises.