fix(cli): route mistral commands to a surface Vibe reads #939

Observed

The mistral profile declares a CommandsCapability installing to .vibe/commands/ and sets signalDir: ".vibe/commands". Its rewriteContent also rewrites command references to .vibe/commands/<phase>/ while buildInstallPath flattens phase directories away, so the rewritten path is never the installed path.

Vibe has no commands directory: its project-local surfaces are skills/, tools/, agents/, prompts/, plugins/, config.toml, hooks.toml, and its slash commands are user-invocable skills. Everything the CommandsCapability installs is invisible to Vibe, and the rewrite/install inconsistency is moot inside a dead directory.

Reproduction

1. Read Vibe's project-local layout reference: no commands/ entry.
2. aidd translate --to mistral (flat) with a plugin that has commands/: files land under .vibe/commands/, which Vibe never scans.

Impact

AIDD commands translated for Mistral Vibe are silently dropped by the tool; the doctor's signal directory points at a dead path.

Evidence

• Vibe's .vibe/ layout reference: no commands directory; user-invocable skills are the slash commands.
• cli/src/contexts/tools/domain/profiles/mistral/profile.ts: CommandsCapability buildInstallPath flattening phase dirs, rewriteContent keeping them.

Suggested direction

Either map AIDD commands to Vibe's user-invocable skills (the design question needs a decision), or drop the capability and repoint the signal directory.

Refs #937 (the PR that ships the mistral profile).