feat: wire OpenCode as a third harness via Yoke

[d180]

Summary

• Add OpenCode as a third harness (alongside Codex/Claude) via almanac-yoke: harness selection, config/model validation, setup wizard, instruction install, and transcript sync/runtime reading.

Why

Yoke (our provider library) added OpenCode support (companion PR: AlmanacCode/Yoke#1). CodeAlmanac's own harness selection, config validation, setup wizard, instruction installer, and transcript sync only knew about Codex/Claude, so OpenCode had no path through any of them.

Verification

uv run pytest        # 513 passed
uv run ruff check .  # clean
git diff --check     # clean
uv build --out-dir dist
uvx twine check dist/*  # PASSED
Also live-tested against a real opencode CLI and real git repos through init, garden, ingest, and sync.

Docs and wiki

• README/docs updated if user-facing behavior changed.
• almanac/ wiki updated if an implementation decision, workflow, invariant, or gotcha changed.
• Not applicable.

Notes for reviewers

• pyproject.toml previously pinned almanac-yoke via a local dev-only path override; that's been removed and repinned to >=0.1.7,<0.2 (a real published version). This PR is still sequenced behind feat: add OpenCode as a third provider adapter Yoke#1 — the one skipped test (test_adapter_forwards_exact_prompt_and_model[opencode]) self-clears once Yoke publishes a release with Provider.OPENCODE.
• default_source_runtime_adapters() had hardcoded its adapter list directly, bypassing an existing aggregator — worth a look since it's an easy place for a future adapter to go missing silently again.