run --output-dir groups files by timestamp

[chlowell]

Today, waza run --output-dir foo writes files like foo/model.json that subsequent runs with the same arguments overwrite. With this PR, run --output-dir foo instead writes files like foo/<timestamp>/model.json. The timestamp has millisecond precision, so it's still possible in principle for one run to overwrite another's results, however I doubt this matters in practice given our expected usage patterns.

[Copilot]

Pull request overview

Updates waza run --output-dir to avoid overwriting prior runs by writing artifacts into a per-run timestamped subdirectory, and adjusts docs/tests accordingly.

Changes:

• Write structured output to {outputDir}/{timestamp}/... instead of directly under {outputDir}.
• Add testable writeOutputDirAt(..., now time.Time) and update tests to validate timestamped layout and non-collision behavior.
• Document the new --output-dir behavior and mutual exclusivity with --output.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
site/src/content/docs/reference/cli.mdx	Documents timestamped subdirectory behavior for --output-dir.
cmd/waza/cmd_run_test.go	Updates output-dir tests to validate new directory structure and repeat-run separation via fixed timestamps.
cmd/waza/cmd_run.go	Implements timestamped run directory via writeOutputDirAt and updates flag help + comments.
README.md	Documents timestamped --output-dir output layout and exclusivity with --output.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@52e123f). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
cmd/waza/cmd_run.go	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #153   +/-   ##
=======================================
  Coverage        ?   73.53%           
=======================================
  Files           ?      141           
  Lines           ?    15861           
  Branches        ?        0           
=======================================
  Hits            ?    11663           
  Misses          ?     3351           
  Partials        ?      847           

Flag	Coverage Δ
go-implementation	73.53% <85.71%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.