Add GitHub Copilot CLI support

[t1gu1]

Why

Added copilot-cli cause some are using it.
I was thinking about adding gemini-cli too, but it is migrating to antigravity-cli so i'll wait for that one.

Added

• GitHub Copilot CLI as an AI provider — copilot-cli joins claude-code-cli, codex-cli, and opencode-cli in the AIProvider union. It piggybacks on the user's locally-installed copilot binary and their GitHub Copilot subscription — no API key required. Detection mirrors the existing CLI providers (binary discovery across PATH + common install locations) and it appears in Settings → AI with the same status/re-detect pattern. Prompts run one-shot via copilot -p (model selectable via the per-provider model picker, free-text since Copilot has no enumeration command). Tool permissions are deliberately restricted (--deny-tool=shell, --deny-tool=write, --no-ask-user, and COPILOT_ALLOW_ALL stripped from the child env) so Copilot only produces text and cannot edit files, run shell commands, or block on interactive prompts.

Technical

• New Rust commands detect_copilot_cli and copilot_cli_prompt (copilot --no-color --deny-tool=shell --deny-tool=write --no-ask-user [--model …] -p <prompt>), plus a CopilotCliInfo type; registered in lib.rs. Mirrored across all three layers: commands/ai.rs, dev-server.mjs, and the backend-ai.ts wrapper.
• useAIProvider adds copilot-cli to CLI_AGENT_PROVIDERS, dispatches it in suggest() / rawPrompt(), and forwards the per-provider model. SettingsPanel gains the provider option, detection state, and status block.
• i18n: aiProviderCopilotCli, aiProviderCopilotCliNotFound, aiCopilotCliDetectedHint, aiCopilotCliInfoBox across all five locales (en, fr, es, pt-BR, zh-CN).
• Unit tests extended in useAIProvider-opencode.test.ts for the Copilot dispatch and model fallback.

[t1gu1]

Please if you can merge that PR first -> #40

I care less about the copilot cli one and it will be easier to fix conflict for me.

Merci 🙏🏻