feat: add Cursor Agent support via --agent flag

[moisei]

Summary

Adds Cursor Agent support to RTK via a new --agent <name> flag on rtk init.

• rtk init -g --agent cursor installs a preToolUse hook in ~/.cursor/hooks.json that rewrites shell commands through rtk rewrite, using Cursor's JSON format ({permission, updated_input})
• Works with both Cursor editor and cursor-cli — they share ~/.cursor/hooks.json, so a single installation covers both
• rtk init --show now reports Cursor hook and hooks.json status alongside existing Claude Code / OpenCode status
• rtk init -g --uninstall also removes Cursor artifacts (hook script + hooks.json entry)
• rtk discover adds a note when Cursor hooks are detected, directing users to rtk gain for Cursor session analytics (Cursor transcripts use a text-only format without structured tool_use/tool_result blocks, so discover cannot extract commands from them)

Design decisions

• --agent is an extensible enum (claude, cursor), not a boolean flag — ready for future agents (Windsurf, Zed, etc.)
• Additive behavior: --agent cursor installs Cursor hooks ON TOP of the default Claude Code installation. Without --agent, behavior is 100% unchanged.
• --opencode stays separate for backward compatibility
• Cursor hook uses preToolUse with matcher: "Shell" and returns updated_input to rewrite commands — functionally identical to Claude Code's PreToolUse but with Cursor's JSON schema

cursor-cli note

The preToolUse hook mechanism is shared between Cursor's editor agent and its CLI agent (cursor-cli). Both read hooks from ~/.cursor/hooks.json, so rtk init -g --agent cursor covers both without any additional setup.

Files changed

File	Change
hooks/cursor-rtk-rewrite.sh	New: Cursor-format hook script (thin delegator to rtk rewrite)
src/main.rs	Add --agent <name> flag with AgentTarget enum
src/init.rs	Cursor install/uninstall/show functions, hooks.json patching, unit tests
src/discover/provider.rs	Doc comment update noting Cursor limitation
src/discover/report.rs	Cursor note in discover output when hook detected

Test plan

• cargo build — 0 errors
• cargo test — 899 passed, 0 failed
• rtk init --help shows --agent with claude and cursor values
• rtk init -g --agent cursor --auto-patch installs hook + patches hooks.json
• rtk init --show reports Cursor hook status
• rtk init -g --uninstall removes Cursor artifacts
• Cursor agent session with hook active rewrites git status → rtk git status
• rtk gain --history shows tracked commands from Cursor session