fix(acp): normalize OpenCode command results

[ymichael]

What was wrong

OpenCode carries command output and exit status in its vendor-specific rawOutput.output and rawOutput.metadata fields. The shared ACP command-result parser intentionally recognizes only standardized content and established generic result shapes, so completed OpenCode command rows could lose their output and exit code.

What changed

Added an OpenCode ACP dialect that validates the recorded result envelope and normalizes output, metadata.output, and metadata.exit only when interpreting an OpenCode command. Standard ACP content, Cursor-style streams, grok's result fields, and generic ACP behavior retain precedence. The built-in OpenCode provider now declares the dialect, and the custom-agent configuration guide and bb-cli skill list opencode as a supported dialect. There are no host-daemon wire-contract changes, so HOST_DAEMON_PROTOCOL_VERSION is unchanged.

How you verified

• Added regression coverage for the recorded OpenCode envelope, metadata output fallback, standardized-content precedence, established result-shape precedence, and generic ACP isolation.
• pnpm exec turbo run test --filter=@bb/provider-bridge-acp -- src/dialect.test.ts src/delta-translation.test.ts
• pnpm exec turbo run typecheck --filter=@bb/provider-bridge-acp --filter=bb-plugin-provider-acp
• pnpm exec turbo run test --filter=@bb/provider-bridge-acp --filter=bb-plugin-provider-acp

Fixes: N/A (no linked issue)

AGENT GENERATED