fix(init): make --agent cursor truly standalone (#213)

[pszymkowiak]

Summary

• rtk init -g --agent cursor no longer attempts to install Claude Code artifacts (RTK.md, CLAUDE.md patch, settings.json hook).
• ~/.cursor/ is now auto-created when missing, so Cursor-only users without a prior config directory don't crash.
• Adds regression tests covering both fixes.

Context

Issue #213 originally asked for Cursor support. PR #595 added --agent cursor (merged 2026-03-18), but the routing logic kept install_claude = !opencode, ignoring the explicit --agent selection. Cursor-only users hit:

```
rtk: Failed to write RTK.md: ~/.claude/RTK.md: Failed to create temp file in ~/.claude: No such file or directory (os error 2)
```

Reported in issue #213 by @ayhangokhan and re-confirmed by @cgondrovic-servpro ("why is it referencing anything claude related?").

Changes

• `should_install_claude(opencode, agent)` helper in `src/main.rs`:
  • `opencode=false, agent=None` → `true` (default `rtk init -g`)
  • `opencode=true` → `false` (OpenCode plugin only)
  • `agent=Some(Claude)` → `true` (explicit opt-in)
  • `agent=Some(Cursor|Windsurf|Cline|Kilocode|Antigravity)` → `false`
• `patch_cursor_hooks_json` creates the parent `~/.cursor/` on demand via `fs::create_dir_all`.
• Comment updated: Cursor hooks are standalone when `--agent cursor` is the only target, additive when Claude Code is also being installed.

Test plan

• `cargo test --all` — 1827 passed, 0 failed
• `cargo clippy --all-targets -- -D warnings` — clean
• `cargo fmt --all --check` — clean
• Manual e2e (empty HOME): `HOME=/tmp/empty rtk init -g --agent cursor` → exit 0, only `/.cursor/hooks.json` created, no `/.claude/` files.
• Manual e2e (pre-existing .claude/): same command with `mkdir /.claude` first → exit 0, `/.claude/` left untouched.
• New unit test `test_should_install_claude_routing` exhaustively covers the (opencode, agent) matrix.
• New unit test `test_patch_cursor_hooks_json_creates_missing_parent_dir` confirms auto-creation.

Closes #213.

🤖 Generated with Claude Code

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