feat: add OpenCode plugin support

[zeval]

Warning

Depends on #241 which streamlines the rewrite logic so that hooks don't have to repeat code

Summary

Add OpenCode plugin support to RTK. The plugin uses OpenCode's tool.execute.before hook to rewrite Bash/Shell tool commands to rtk ... before execution.

Built on top of PR #241 (rtk rewrite): the OpenCode plugin is a thin 38-line wrapper that delegates all rewrite logic to rtk rewrite.

What's added

• hooks/opencode-rtk.ts (38 lines) — OpenCode plugin that calls rtk rewrite via shell
• rtk init -g --opencode — install the plugin to ~/.config/opencode/plugins/rtk.ts
• rtk init -g — now installs both Claude Code hook AND OpenCode plugin by default
• rtk init -g --claude / --opencode — install only one integration
• rtk init --show — reports OpenCode plugin status
• rtk init -g --uninstall — removes OpenCode plugin alongside Claude Code artifacts
• Docs: README section, troubleshooting guide

Architecture

Claude Code hook   55-line bash  ─┐
OpenCode plugin    38-line TS    ─┤  rtk rewrite "cmd"
Future hooks       ~15 lines    ─┘  (single source of truth)
                                     └── src/rewrite_cmd.rs
                                     └── src/discover/registry.rs

Both hooks are thin protocol adapters. All rewrite logic lives in Rust.

Design decisions

• --opencode as opt-in flag: `rtk init -g` installs both by default; `--claude` or `--opencode` installs only one
• Plugin directory: `~/.config/opencode/plugins/` per OpenCode docs
• No parity test needed: both hooks delegate to `rtk rewrite`, so there's nothing to drift
• Known limitation: OpenCode plugin hooks don't intercept subagent tool calls (upstream issue)

Files changed

File	Change
`hooks/opencode-rtk.ts`	New — 38-line thin wrapper calling `rtk rewrite`
`src/init.rs`	Add OpenCode install/update/uninstall/status (+243/-26)
`src/main.rs`	Add `--claude`/`--opencode` flags to Init command
`README.md`	Add OpenCode setup section
`docs/TROUBLESHOOTING.md`	Add OpenCode troubleshooting

Testing

cargo test --all                       # 476 tests pass
cargo run -- init -g --opencode        # installs plugin
cargo run -- init --show               # shows OpenCode status
cargo run -- init -g --uninstall       # removes all artifacts

Closes

• Closes add support/installation for OpenCode #117

[zeval]

Rewrote this to respect the work done in #241 since we don't want to keep adding repeated re-write logic. When #241 is merged, this should be ready as well.