ci: split tests into 4 shards with fail-fast disabled

[yanyihan-xiaomi]

Summary

• Split unit tests into 4 parallel shards using bun test --shard
• Set fail-fast: false so all shards run to completion even if one fails
• Run tests directly in packages/opencode instead of via turbo
• Remove @mimo-ai/cli and @mimo-ai/app test tasks from turbo.json (only TUI is actively developed)
• turbo.json only keeps typecheck
• Pin bun to 1.3.14 via packageManager field
• Upload JUnit XML artifact per shard (if: always())
• Bump actions: checkout@v6, cache@v5, upload-artifact@v7 (fix Node.js 20 deprecation warnings)
• Remove RUNNER_ARCH guard in setup-bun (CI is ubuntu-only)

Current status

• Sharding works correctly: 4 independent shards, fail-fast disabled confirmed
• Shard 3/4 passes fully
• 21 pre-existing test failures across shards 1/2/4 (unrelated to this PR):
  • Shard 1: 9 fail (History.backfill, lsp.spawn, MCP server, Worktree ×2, rebuild context ×2, bash permissions ×2)
  • Shard 2: 2 fail (Project.fromDirectory, rebuild context visual verify)
  • Shard 4: 10 fail (Actor.spawn, plugin specs ×3, migrateFromGlobal, SessionCheckpoint, CheckpointContext, parentSessionID, Instruction.system ×2)

Test plan

• Verify all 4 shards start and run independently
• Verify a failing shard does not cancel other shards
• JUnit artifacts uploaded per shard

[yanyihan-xiaomi]

Superseded by #1236