feat: add OpenCode 2 and MiMo ACP providers

[giuliastro]

Summary

Adds two more real ACP harness profiles through the Provider Kit path, stacked on #536. This PR targets feature/acp-provider-copilot, never main.

OpenCode 2

• provider id: opencode2
• detection: optional dedicated opencode2 binary alias
• launch: uses opencode2 acp when that alias exists, otherwise falls back to npx --yes --package=@opencode/cli opencode acp so v2 can be tested/used without replacing the current opencode binary
• models: enabled
• reasoning/effort variant hint: effort
• runtime commands: enabled
• permissions: enabled
• existing OpenCode HTTP integration remains separate as opencode

MiMo Code

• provider id: mimo
• detection: mimo
• launch: mimo acp
• conservative initial capability surface because current MiMo ACP releases have open compatibility issues around model defaults and Session prompting
• provider-driven authenticate: false: current MiMo ACP advertises opencode-login from initialize but its own authenticate implementation throws Authentication not implemented; Harness Remote therefore relies on credentials already configured in MiMo CLI
• core Session lifecycle is real-machine validated on MiMo Code 0.1.14 against a real workspace
• models/commands remain disabled until separately validated

Provider Kit / launcher

• provider discovery and priority are registry-driven
• launcher help/error text now derives supported backend ids from the registry instead of hardcoded names
• existing primary order remains unchanged: Codex → Claude → OMP → PI → Copilot → OpenCode 2 → MiMo

Real-machine smoke commands

Both smokes use a temporary workspace by default and run through Harness Remote's real AcpService:

npx --yes --package=github:giuliastro/harness-remote#feature/acp-provider-opencode2-mimo harness-remote-opencode2-smoke
npx --yes --package=github:giuliastro/harness-remote#feature/acp-provider-opencode2-mimo harness-remote-mimo-smoke

The OpenCode 2 smoke also checks runtime commands and model config options. MiMo starts with lifecycle/prompt/Stop/reopen checks only. Its smoke defaults to the caller's current workspace because MiMo 0.1.14 can reject session/new in an empty temporary directory even though the same ACP flow passes in a real workspace.

Stack

• refactor: introduce ACP Provider Kit foundation #533 Provider Kit foundation
• refactor: drive ACP launcher detection from provider metadata #534 provider-driven detection
• test: add ACP provider lifecycle contract fixtures #535 lifecycle contract fixtures
• feat: add GitHub Copilot CLI ACP provider #536 GitHub Copilot provider
• this PR: feature/acp-provider-opencode2-mimo

Nothing in this stack is merged to main.

Related: #372