fix(cli): merge explorer options from config file

[kariy]

The [explorer] section in katana.toml was being ignored because NodeArgsConfig was missing the explorer field and with_config_file() didn't merge it. This adds the missing field and merge logic so that explorer = true in the config file now works the same as the --explorer CLI flag.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.60%. Comparing base (9bde0ae) to head (dfd4f2b).
⚠️ Report is 217 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/cli/src/file.rs	0.00%	3 Missing ⚠️
crates/cli/src/args.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #384      +/-   ##
==========================================
+ Coverage   73.32%   73.60%   +0.28%     
==========================================
  Files         209      257      +48     
  Lines       23132    33151   +10019     
==========================================
+ Hits        16961    24402    +7441     
- Misses       6171     8749    +2578     

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