fix(cli): apply scale down in JSON mode

[mhb888248]

Problem

sh1pt scale down --json printed a successful removal result and returned before mutating fleet state. Automation therefore saw a removed instance even though credentials.json still contained the full fleet.

Fix

• compute one result payload for both dry-run and executed JSON output
• persist the filtered fleet before returning JSON in normal mode
• keep --dry-run --json non-mutating
• add a subprocess regression test that checks the saved fleet and preserves unrelated credentials

Verification

• pnpm exec vitest run packages/cli/src/commands/scale.test.ts (59 passed)
• pnpm --filter @profullstack/sh1pt... build
• pnpm --filter @profullstack/sh1pt typecheck
• isolated CLI reproduction confirmed two instances become one while the API key remains unchanged