feat: replace Claude Code with OpenCode

[dtump]

Summary

• Replace the pinned Claude Code CLI with pinned OpenCode while preserving lifecycle-script safeguards.
• Migrate wrapper/config mounts, persistent state, CI, tests, and documentation to OpenCode.
• Add a migration section for claude-docker users.

Validation

• python3 -m unittest discover -s tests -p "test_*.py" -q
• Shell syntax checks for wrapper, entrypoint, smoke, and integration scripts.
• CI workflow YAML parsing.
• docker build -t opencode-docker:validation .
• docker run --rm opencode-docker:validation opencode --version (1.18.16)
• IMAGE=opencode-docker:validation bash smoke/smoke.sh --uid="$(id -u)" --volstate=cold --ephemeral=0