stash CLI: eql install shouldn't require a stash.config.ts; support standalone --database-url + document URL precedence #581

Background

Follow-up design work that came out of the #578 / #579 fixes. eql install coupled EQL installation to scaffolding-and-loading a stash.config.ts, which is what produced the standalone-npx crash (#579). But eql install only ever needs a database URL — it reads nothing else off the config that isn't otherwise available (config.databaseUrl + a client path that has a default).

Enhancement

Decouple eql install from the config, and make the database URL sourcing explicit and documented.

• Config is optional for eql install. Resolve the URL directly (--database-url → DATABASE_URL → supabase status → prompt) via resolveDatabaseUrl. A standalone npx stash eql install --database-url ... then works in a bare project with zero dependencies. An existing stash.config.ts is still authoritative (later commands rely on it); without one, a config is offered as a convenience, not required.
• --database-url is a one-shot signal. When passed explicitly, eql install leaves the project untouched — no stash.config.ts and no encryption client are scaffolded.
• Document the resolution precedence. Make the order discoverable: on resolveDatabaseUrl (JSDoc), in stash manifest (registry descriptor), and in stash --help. Order (first wins): --database-url flag → DATABASE_URL env → supabase status → interactive prompt. stash.config.ts is not a separate tier — its default databaseUrl re-runs the same chain.

Status

Implemented in #580 (alongside the #578 / #579 fixes).