Add OpenCode as a built-in ACP runtime

[ral2f]

Summary

• promote OpenCode from a preset to a built-in ACP runtime launched as opencode acp
• expose OpenCode's ACP-advertised account login without making authentication a readiness requirement
• preserve provider-qualified model discovery and selection through the existing ACP paths
• add backend, frontend, auth, catalog, mapping, and logo regression coverage

Verification

• just ci (final commit ea2688407dbaf872fa1556e5e136540185de5314)
• focused Rust discovery suite: 103 passed
• desktop frontend suite: 3,874 passed
• desktop Rust suite: 2,007 passed, 14 ignored; 3 integration tests passed
• mobile suite: 1,022 passed, 1 skipped
• TypeScript, Biome/file-size gates, Rust fmt/Clippy, desktop/web builds, Flutter analyze all passed

Notes

• Buzz does not read or store OpenCode credentials.
• OpenCode Go models remain owned and returned by OpenCode after login; no model IDs are hard-coded.
• No agent-config rules changed.

[Chessing234]

solid. worth calling out in the pr how this overlaps with #3868 / openclaw so reviewers know which runtime catalog to land first

[innerline]

Alternative implementation now up as #6526 (innerline:feat/opencode-runtime). Complementary rather than conflicting — this PR's account-connection surface and provider-qualified model discovery are things #6526 does not have; #6526 covers the operational side this one leaves out: auto-install (underlying_cli + vendor installer command, so the Desktop install button works), .opencode/skills skill discovery, deterministic ~/.opencode/bin PATH probing (the installer only edits rc files), a BUZZ_DEFAULT_RUNTIME default-engine override, and a spec-conformant remote-agent image example (examples/sprig-opencode, image-override pattern per remote-agents.md). Also noting two verified-against-source Nones in #6526's runtime entry: no faithful auth probe exists today (opencode auth list exits 0 with zero credentials) and opencode core reads no model env var. Whichever direction maintainers pick, the other's pieces are worth harvesting.