feat: support opencode zen reviewer in --review-with

[atomantic]

Summary

• Adds support for opencode as a local-agent reviewer under --review-with (with friendly aliases zen and opencode-zen).
• Configures default Zen model to opencode/muse-spark-1.3-contributor-free.
• Normalizes friendly model identifiers like muse-1.3, zen/muse-1.3, muse-spark-1.3, and bare names to installed Zen models.
• Maps ~effort=<level> to OpenCode's native reasoning effort flag --variant <level>.
• Runs OpenCode headlessly in pure isolation mode (opencode run --pure) with redirected stdin (< /dev/null) to ensure non-interactive execution.
• Updates documentation, CLI command definitions (do:pr, do:release, do:review, do:rpr, do:config, do:depfree, do:better-swift, do:pr-better), and library partials.
• Adds test coverage verifying OpenCode reviewer parsing, model resolution, effort mapping, and do:config round-tripping.

Test Plan

• Ran node --test test/review-loop-contract.test.js test/config.test.js (all tests pass).
• Ran full test suite via npm test (524 passed, 0 failed).