Add OpenCode Go provider support and bring repo up to ecosystem convention

[alpibrupa]

Summary

lex-code was three weeks stale relative to the rest of the lex ecosystem and lacked native support for the OpenCode Go plan that lex-loom already uses.

OpenCode Go support (native + via the LiteLLM proxy, matching lex-loom)

• New opencode_agent() in all seven src/agents/*.lex files (build/explore/plan/refactor/review/spec_agent/test_agent), using lex-llm's existing providers.opencode_go() — no new package code needed there. Since the Go plan serves full-scale cloud models (DeepSeek, Qwen3, Kimi, GLM, MiniMax, MiMo), these get the same full tool budget as the other cloud providers (mistral/openai/google), not litellm/ollama's stripped-down local-model treatment.
• tools.opencode_model() (src/tools/index.lex) reads OPENCODE_MODEL, mirroring the existing litellm_model() pattern.
• Wired into src/server/session.lex's pick_agent and src/tui/main.lex's select_provider_tag/help text as a new --opencode flag.
• litellm/config.yaml + litellm/docker-compose.yml: lex-code had no bundled proxy config at all — the README referenced a litellm_config.yaml at project root that didn't exist. Copied from lex-loom's own litellm/ directory — same model list including the OpenCode Go entries (DeepSeek/Qwen3/Kimi/GLM/MiniMax/MiMo) — so both repos can point at one shared proxy instance, and --litellm now works out of the box.
• README: new "OpenCode Go plan" section, provider table row, and LiteLLM section pointing at the bundled config instead of a nonexistent file.

Housekeeping (bringing this repo to the same convention as lex-agent/lex-soft/lex-loom/lex-memory)

• lex.toml: bump the lex version pin from 0.9.7 to 0.10.9 (CI itself already installs 0.10.7; the rest of the ecosystem is on 0.10.9), add license = "EUPL-1.2" and a description.
• LICENSE file (matches lex-lang/lex-os/lex-soft/lex-loom/lex-agent/lex-memory's EUPL-1.2 notice-and-pointer form).
• AGENTS.md + CLAUDE.md: this repo had neither, despite being the project that builds the coding-agent convention for everyone else. Documents the two-call-sites-per-provider pattern (agents/*.lex + session.lex's pick_agent), why there's no tests/ directory (CI gates on per-file lex check + the manifesto demos), and the local-model tool-budget curation.
• README: License section, v0.6 roadmap entry.

lex-code is not being wired into lex-loom as a dependency (discussed separately) — loom's own agent runner already does the equivalent job, deliberately vendored rather than imported, the same isolation call loom makes for lex-soft's runtime.

Verification

lex pkg install                        # resolves cleanly (12 packages)
lex check <every tracked src/*.lex>    # matches CI's own loop — all ok
lex fmt --check src/                   # all 76 files formatted
lex check examples/manifesto_parallel.lex          # passes
lex check examples/manifesto_parallel_bad.lex      # correctly rejected
bash examples/manifesto_semantic_diff/run.sh        # validates

Live smoke test (script not part of this diff): two lex run invocations confirm build_agent.opencode_agent() constructs and session.pick_agent(Build, "opencode") dispatches to it correctly.

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