fix(reporters): use timestamped filenames for JSON/CSV output

[boneskull]

Default output filenames for JSON and CSV reporters are now uniquely named
with timestamps (e.g., benchmarks-2025-12-17-10-30-45.json) instead of the
static 'results.json' / 'results.csv'. This prevents accidental overwrites
when running multiple benchmark runs to the same output directory.

Closes #196

[Copilot]

Pull request overview

This PR changes the default output filenames for JSON and CSV reporters from static names (results.json, results.csv) to timestamped filenames (e.g., benchmarks-2025-12-17-10-30-45.json). This prevents accidental overwrites when running multiple benchmark runs to the same output directory and addresses issue #196.

Key changes:

• Added generateTimestampedFilename() function that creates filenames with timestamp format benchmarks-YYYY-MM-DD-HH-MM-SS.{extension}
• Updated JSON and CSV reporter setup to use timestamped filenames as defaults instead of static names
• Added findFileByPattern() test utility to locate timestamped output files in integration tests

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
src/core/output-path-resolver.ts	Adds generateTimestampedFilename() function to create timestamped filenames with format benchmarks-YYYY-MM-DD-HH-MM-SS.{extension}
src/cli/commands/run.ts	Updates CSV and JSON reporter initialization to use generateTimestampedFilename() instead of static results.csv and results.json
test/util.ts	Adds findFileByPattern() helper function to locate timestamped output files in test directories
test/unit/output-path-resolver.test.ts	Adds comprehensive unit tests for generateTimestampedFilename() including format validation, extension handling, and zero-padding verification
test/integration/reporters.test.ts	Updates tests to use findFileByPattern() for locating timestamped output files; updates conflict test to use explicit --output-file
test/integration/verbose-mode.test.ts	Updates verbose mode tests to find and verify timestamped JSON and CSV output files
test/integration/quiet-mode.test.ts	Updates quiet mode tests to locate and verify timestamped output files

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