feat(opencode): support project-local plugin install

[jjangga0214]

Summary

• allow rtk init --opencode to install .opencode/plugins/rtk.ts in the current project
• keep rtk init -g --opencode as the existing global install path
• update uninstall, rtk init --show, help text, and docs to reflect both install scopes

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Manual testing: rtk init --opencode, rtk init --show, and rtk init --opencode --uninstall output inspected

Notes:

• cargo build succeeded with Rust 1.95.0.
• cargo test --all passed locally.
• cargo clippy --all-targets completed locally and reports existing repository warnings unrelated to this change.

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✅ jjangga0214
✅ aeppling
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