fix(cli): make the mistral marketplace route detectable by Vibe #938

Observed

aidd translate builds two targets for Mistral Vibe. The flat route materializes .vibe/skills/ directly and was proven on a real Vibe install. The marketplace route emits a Claude-style native plugin tree with its manifest at .vibe-plugin/plugin.json, and the profile's distributionProbes assert that same path.

Vibe's documented native plugin format puts plugin.json at the plugin root (Agent Plugins 1.0), and its foreign-format adapters recognize only .claude-plugin/, .codex-plugin/, .kimi(.|-)plugin/, and .opencode/ markers. .vibe-plugin/plugin.json matches neither, so Vibe never detects the tree and the probes can never fire.

Even if detection were fixed, parts of the tree would stay dead: Vibe agents are TOML under ai.mistral.vibe/agents/, not markdown, and foreign-adapted plugins contribute skills and MCP only.

Reproduction

1. aidd translate --to mistral in marketplace mode.
2. Inspect the output: manifest at .vibe-plugin/plugin.json, agents as agents/*.md.
3. Compare against Vibe's plugin format reference.

Impact

The marketplace route for Mistral Vibe ships files nothing reads. The flat route is the only install-verified path.

Evidence

• Vibe's plugin system reference: native manifest plugin.json at plugin root; foreign-format table; foreign plugins contribute skills and MCP only.
• cli/src/contexts/tools/domain/profiles/mistral/mistral-paths.ts: OUTPUT_MISTRAL_MANIFEST_RELATIVE = ".vibe-plugin/plugin.json".
• The route was never exercised against a real Vibe install; only the flat route has one on record.

Refs #937 (the PR that ships the mistral profile; the flat route there is the proven path).