Add seconds to auto-generated run directory names

[cweniger]

Summary

• Changes the auto-generated output directory format from YYMMDD-HHMM-adj-noun to YYMMDD-HHMMSS-adj-noun
• Removes a redundant datetime.now() call (single call instead of two)
• Updates the docstring in cli.py to reflect the new format

Motivation

The minute-precision format caused collisions when multiple runs were launched within the same minute. Adding seconds makes each run name unique under normal usage.

Test plan

• falcon launch without --output produces a directory named output/YYMMDD-HHMMSS-adj-noun
• Re-running within the same minute produces a distinct directory

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 9.61%. Comparing base (7179199) to head (f97a86a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
falcon/core/run_name.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #86   +/-   ##
=====================================
  Coverage   9.61%   9.61%           
=====================================
  Files         33      33           
  Lines       4141    4141           
=====================================
  Hits         398     398           
  Misses      3743    3743           

Flag	Coverage Δ
unit	9.61% <0.00%> (ø)

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