fix(init): keep cursor setup out of claude config

[ousamabenyounes]

Summary

• Treat Cursor init as a Cursor-only setup path instead of running Claude setup first.
• Ensure the Cursor config directory exists before atomically writing hooks.json.

Test verification (RED -> GREEN)

RED on upstream-base behavior with the new regression test:

called `Result::unwrap()` on an `Err` value: Failed to write RTK.md: /tmp/.tmpGNCwuf/.claude/RTK.md
Caused by:
    0: Failed to create temp file in /tmp/.tmpGNCwuf/.claude
    1: No such file or directory (os error 2)
test hooks::init::tests::test_cursor_global_init_does_not_require_claude_dir ... FAILED

GREEN after the fix:

Cursor hook registered (global).
hooks.json: RTK preToolUse entry added
test hooks::init::tests::test_cursor_global_init_does_not_require_claude_dir ... ok

Full local CI replay:

cargo fmt --all -- --check
cargo clippy --all-targets -- -D warnings
cargo test --all
test result: ok. 2512 passed; 0 failed; 8 ignored; 0 measured; 0 filtered out

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all
• Manual testing: rtk init --global --agent cursor behavior covered by regression test

Fixes #3282

Important: All PRs must target the develop branch (not master).