fix: list --format json outputs [] for empty collections

[hummer98]

Summary

• firex list <path> --format json did not produce valid JSON for an empty collection: it printed the human-readable text No matching documents found (or nothing at all with --quiet) instead of [].
• OutputFormatter.formatDocuments already returns '[]' for an empty array (see src/presentation/output-formatter.ts), so the non-empty JSON path was already correct — the bug was that ListCommand#queryCollection short-circuited on an empty result before reaching that formatter, printing text unconditionally instead.

Root cause

In src/commands/list.ts, queryCollection had:

if (documents.length === 0) {
  if (!quiet) {
    console.log('No matching documents found');
    console.log(`\nExecution time: ${executionTimeMs}ms`);
  }
  return;
}

This branch ran regardless of --format, so --format json on an empty collection either printed nothing (--quiet) or the text message — neither of which is parseable JSON.

Fix

Check format === 'json' first in the empty-result branch and print the literal [] (matching the existing convention already used by OutputFormatter.formatDocuments for non-empty JSON output), regardless of --quiet. Non-JSON formats (yaml/table/toon) keep their existing "No matching documents found" text behavior unchanged — only the JSON contract is fixed.

Tests

Added to src/commands/list.test.ts:

• --format json + empty collection, quiet=false → stdout is exactly []
• --format json + empty collection, quiet=true → stdout is exactly []
• --format json + non-empty collection → unchanged (regression)
• non-json format + empty collection, quiet=false → still prints No matching documents found (unchanged)
• non-json format + empty collection, quiet=true → no output (unchanged)

src/mcp/tools/list.ts (MCP tool) was checked and does not share this bug — it always wraps output in { count, executionTimeMs, documents: [...] }, which is valid JSON even when documents is empty.

Local verification

• npm run typecheck — pass
• npm test — 1044/1044 tests pass (59 files)
• npm run build — pass
• npm run lint — could not verify: this repo has no ESLint config file anywhere in git history (confirmed via git log --all --diff-filter=A -- '*eslintrc*' 'eslint.config*'), so npm run lint fails with "ESLint couldn't find a configuration file" on a clean checkout of master itself, unrelated to this change.

Note on CI status (pre-existing, unrelated issue)

CI on master has been failing for several pushes now (including the 0.9.0 and 0.9.1 release commits) at the Install dependencies step (npm ci) with:

npm error Invalid: lock file's picomatch@2.3.2 does not satisfy picomatch@4.0.4

This is a pre-existing package-lock.json/peer-dependency issue orthogonal to this fix (reproduced on a pristine master checkout without legacy-peer-deps). This PR's CI run is expected to hit the same failure. Flagging it here rather than bundling an unrelated lockfile fix into this PR.

Fixes #6