feat: add project-scoped OpenCode support

[davidrudduck]

Summary

• register OpenCode as a Zenith provider with opencode acp execution and build-mode ACP session setup
• add project-scoped .opencode/opencode.json MCP host configuration with preservation, idempotency, diagnostics, and conflict handling
• add OpenCode agent assets, shared skill installation support, package metadata, and PR-A test coverage
• add audience-specific OpenCode documentation for users, administrators, and developers under docs/opencode-support/

Scope

This PR is limited to OpenCode support PR A: provider registration, project-scoped OpenCode host config, OpenCode agents/skills, documentation, and hermetic/disposable validation. It intentionally does not implement PR B model/effort session options, live OpenCode worker smokes, user-scoped setup, provider billing grants, or PR C terminal-review isolation.

Validation

• uv run ruff check .
• uv run mypy src
• uv run pytest -q -rs -> 283 passed, 7 skipped
• uv build
• focused OpenCode/docs follow-up validation: uv run pytest -q tests/test_cli.py tests/test_config.py tests/test_assets.py tests/test_acp_runner.py -> 110 passed
• disposable OpenCode e2e: zenith init --agent opencode twice, byte-idempotent .opencode/opencode.json, opencode debug config resolves mcp.zenith, opencode mcp list reports zenith connected
• root opencode.json conflict e2e: init fails clearly, root config hash preserved, remediation text present, canary values not leaked

Notes

• The 7 skipped tests are opt-in live-provider smoke tests that require explicit env flags/credentials.
• OpenCode validation used local opencode CLI surfaces and did not run paid/live provider dispatch.
• PR-B/PR-C behavior is documented as future scope rather than implemented here.