Opencode feature

[jeanvetorello]

Summary

Add OpenCode feature integration with clean branch history for PR review.

Changes

• Added OpenCode-related runtime integration and streaming flow updates.
• Updated provider/model handling paths required by OpenCode.
• Updated related docs for CLI/config/provider usage.
• Fixed lint issues found during validation:
  • needless borrow in LINE base64 encode path
  • duplicated if/else branch in OpenCode stream completion
  • removed unused stream terminal tracking variable

Testing

• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace passes
• Live integration tested (if applicable) (N/A for this PR)

Additional checks run:

• cargo build --workspace passes
• cargo build --profile release-fast -p openfang-cli passes
• cargo test -p openfang-kernel passes
• cargo test -p openfang-runtime passes
• cargo test -p openfang-memory passes
• cargo fmt --all --check passes

Security

• No new unsafe code
• No secrets or API keys in diff
• User input validated at boundaries

[Jengro777]

https://github.com/librefang/librefang