feat(agents): add opencode CLI agent plugin

[Meirtz]

What

plugins/agents/opencode → agentix.agents.opencode, a sandbox-side wrapper around the opencode terminal coding agent, following the claude-code plugin shape.

• run(instruction, *, workdir, timeout, model, extra_args, env) -> Result invokes opencode run non-interactively and returns exit_code / stdout / stderr. Provider-agnostic — the caller wires the LLM endpoint via env / extra_args (e.g. an OpenAI-compatible base URL from the in-sandbox bridge), per the integrate-an-agent guide.
• default.nix ships the opencode binary on /nix/runtime/bin. The derivation is lifted from numtide/llm-agents.nix (packages/opencode, v1.15.12) and adapted to plain nixpkgs: wrapBuddy → stock autoPatchelfHook, and libstdc++ placed on LD_LIBRARY_PATH for opencode's dlopen'd @parcel/watcher native addon. Multi-platform (x86_64 + aarch64), both hashes from upstream.
• dependencies = ["agentixx"] only — the binary comes via Nix, so the workspace lock gains one additive entry with zero version changes (no pollution).

Verification

• ✅ ruff check
• ✅ pyright (0 errors, full include set; opencode wired into [tool.pyright])
• ✅ uv lock — additive only (Added agentix-agent-opencode, no downgrades)
• ✅ default.nix build validated end-to-end: agentix build plugins/agents/opencode --platform linux/arm64 builds the bundle (nix builds the derivation, fetches + autoPatchelfs the binary).
• ⚠️ unit test — present (tests/test_opencode.py: argv/env/timeout) but not run in CI: the agentix.agents.* namespace only imports from a built bundle, not the editable venv (same as the existing agent plugins).

Wired into [tool.uv.workspace] (via the plugins/agents/* glob) and [tool.pyright].

[Meirtz]

Validated: agentix build plugins/agents/opencode --platform linux/arm64 completed successfully (exit 0) — nix builds the derivation, fetches + autoPatchelf's the opencode binary, and the bundle exports cleanly. Marking ready for review.

[FatPigeorz]

@copilot resolve the merge conflicts in this pull request

[Meirtz]

Rebased onto master (9181aa7). Conflicts were confined to the root pyproject.toml pyright include/extraPaths lists (kept both the qwen-code and opencode entries, sorted) and uv.lock (relocked — the only delta is the agentix-agent-opencode member entry).

Verified locally on the rebased branch: ruff check . clean, pyright 0 errors, pytest tests/ plugins/ 368 passed.

One pre-existing gap intentionally not folded into the rebase: like qwen-code, run() passes cwd=workdir without creating it first — claude-code gained os.makedirs(args.workdir, exist_ok=True) in 6104961. Will send a small follow-up covering both agents.