fix(init): honor OPENCODE_CONFIG_DIR for OpenCode plugin path

[cavanaug]

This PR was generated using AI on behalf of @cavanaug.

Summary

• Fix rtk init -g --opencode to honor OPENCODE_CONFIG_DIR when resolving the OpenCode plugin install directory.
• Preserve existing fallback behavior to ~/.config/opencode when OPENCODE_CONFIG_DIR is unset or blank.
• Add tests covering env override, blank-env fallback, and plugin path preparation using the override.
• Update OpenCode docs/examples to use ${OPENCODE_CONFIG_DIR:-$HOME/.config/opencode}.

Test plan

• cargo fmt --all --check
• cargo clippy --all-targets (2 pre-existing warnings in unrelated files)
• cargo test (982 passed)
• cargo build --release
• cargo test test_resolve_opencode_dir_uses_env_override
• cargo test test_resolve_opencode_dir_falls_back_when_env_is_blank
• cargo test test_prepare_opencode_plugin_path_uses_env_override
• Manual: verify docs path examples are env-aware

[Copilot]

Pull request overview

Updates RTK’s OpenCode integration so rtk init -g --opencode installs the plugin under an OPENCODE_CONFIG_DIR override when provided, while preserving the existing ~/.config/opencode fallback and updating docs accordingly.

Changes:

• Add OPENCODE_CONFIG_DIR support to OpenCode config-dir resolution (blank values fall back).
• Add unit tests for env override, blank-env fallback, and plugin-path preparation using the override.
• Update README + troubleshooting docs to show env-aware paths (${OPENCODE_CONFIG_DIR:-$HOME/.config/opencode}).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/init.rs	Honor OPENCODE_CONFIG_DIR for OpenCode plugin install path + add tests for override/fallback behavior.
docs/TROUBLESHOOTING.md	Use an env-aware path example when checking for the plugin file.
README.md	Document the env override behavior and update manual install commands to use ${OPENCODE_CONFIG_DIR:-...}.

Comments suppressed due to low confidence (1)

src/init.rs:1538

• The doc comment for opencode_plugin_path is now misleading: the plugin path is not always ~/.config/opencode/... because resolve_opencode_dir() can return OPENCODE_CONFIG_DIR. Update the comment to describe the path as $OPENCODE_CONFIG_DIR/plugins/rtk.ts when set, otherwise the ~/.config/opencode/... fallback.

/// Return OpenCode plugin path: ~/.config/opencode/plugins/rtk.ts
fn opencode_plugin_path(opencode_dir: &Path) -> PathBuf {
    opencode_dir.join("plugins").join("rtk.ts")

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