feat: Add OpenCode as first-class native agent

[jdistler]

Summary

Adds OpenCode as a first-class native agent across Happy (happy-cli, happy-agent, and happy-app), resolving #265 and implementing the proposal in #1631 with scope guidance from @bra1nDump:

• Pure ACP protocol over stdio (opencode acp)
• First-class native app support: dedicated agent picker icon, model modes, permission modes, and remote spawn from mobile/web
• End-to-end integration verified locally with installed OpenCode CLI (1.18.25)

Closes #265, Closes #1631. cc @bra1nDump

Changes

1. packages/happy-cli

• Added top-level happy opencode CLI subcommand invoking runAcp with { agentName: 'opencode', command: 'opencode', args: ['acp'] }.
• Added opencode to --help usage and examples.
• Updated daemon session spawning (spawnSession in src/daemon/run.ts) to handle options.agent === 'opencode'.
• Updated daemon controlServer.ts and registerCommonHandlers.ts schemas to accept opencode as an agent.
• Updated CLI availability detection in src/utils/detectCLI.ts and src/api/types.ts (opencode: boolean). Added unit tests in detectCLI.test.ts.

2. packages/happy-agent

• Added 'opencode' to SupportedAgent and SUPPORTED_AGENTS in src/machineRpc.ts and src/index.ts.

3. packages/happy-app

• Catalog & Discovery: Added opencode to HARNESS_NAMES, HARNESS_ORDER, and availability checks in sources/utils/harnessCatalog.ts and sources/sync/machineChoices.ts.
• Defaults & State: Added opencode to agentKeys, CODE_AGENT_DEFAULTS, persistence.ts, and ops.ts.
• Modes: Added getOpenCodePermissionModes and getOpenCodeModelModes with custom model mode support in sources/components/modelModeOptions.ts.
• UI & Assets:
  • Added official OpenCode icon assets (icon-opencode.png, @2x.png, @3x.png) to sources/assets/images/.
  • Wired avatar badges in Avatar.tsx and avatarHarness.ts.
  • Added OpenCode to new session picker (app/(app)/new/index.tsx), agent settings (app/(app)/settings/agents.tsx), and composer (AgentInput.tsx).
  • Added translations across all 10 supported locale files.

Verification

• Verified ACP handshake and session/new lifecycle directly against local OpenCode CLI (opencode acp).
• Tested happy opencode end-to-end (spawned session and verified Status: starting -> Status: idle).
• Full unit test suites pass:
  • packages/happy-cli: 87/87 test files (820 tests) passed.
  • packages/happy-app: 106/106 test files (1,121 tests) passed.
• tsc --noEmit typechecks pass cleanly with 0 errors across all packages.