feat(agents): land GitHub Copilot CLI provider

[anxkhn]

What this PR solves

Lands GitHub Copilot CLI as a first-class provider, sitting next to claude, codex, gemini, aider, and opencode. CopilotAgent mirrors the Gemini/OpenCode adapters and drives the verified copilot v1.0.65 non-interactive surface:

• copilot -p "<prompt>" -s --allow-all-tools --no-ask-user [--model] so it never blocks on a permission or ask-user prompt.
• reviewTask adds --output-format json and parses the JSONL stream: the answer is the last assistant.message content, tokens_out sums per-message outputTokens, and the final result line carries sessionId/exitCode. Copilot exposes no prompt tokens, so tokens_in/cached_tokens stay 0 (unmeasured) instead of being faked.
• Wires copilot into the registry, KNOWN_AGENTS, host detection (COPILOT_CLI), model registry, install hints, and the fallback-provider message.

This supersedes #1085, which used wrong flags (--allow-all, --silent) and a guessed OpenAI prompt_tokens schema that never matched copilot output. Pi is intentionally left out to keep this PR atomic.

How to test it

• npm install
• npm run lint (0 errors), npm run lint:syntax, npx prettier --check on changed files (clean)
• npx vitest run tests/agents tests/config tests/brain tests/commands tests/integration (all pass)
• Real CLI smoke (copilot v1.0.65 authed): reviewTask returned ok=true exit=0 output="smoke-ok" tokens_out=36, confirming the JSONL assistant.message/result parsing.

Tests added or modified

• tests/agents/copilot-agent.test.js (new): surface flags, --model, exit codes, JSONL content + token parsing.
• tests/agents/agent-detect.test.js, tests/integration/agents-index-contract.test.js: extend coverage to copilot.

Notes for the reviewer

Net source delta stays under the 200-LOC shrink budget (~194). README and docs are excluded by the gate.

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Qué resuelve este PR

Incorpora la CLI de GitHub Copilot como proveedor de primera clase. CopilotAgent replica los adaptadores Gemini/OpenCode y usa la superficie verificada de copilot v1.0.65: -p con -s --allow-all-tools --no-ask-user [--model], y en reviewTask añade --output-format json para parsear el JSONL (contenido de assistant.message, outputTokens, y la línea result con sessionId/exitCode). Reemplaza los flags y el esquema JSON incorrectos de #1085. Pi se deja fuera para mantener el PR atómico.

Cómo probarlo

npm install, npm run lint, tests de agentes/config/brain/commands/integration, y smoke real con la CLI.

Tests añadidos o modificados

copilot-agent.test.js nuevo, mas extensiones en agent-detect e index-contract.

Notas para el revisor

Delta neto por debajo del presupuesto de 200 LOC.

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