cli: migrate from pico-args to clap 4.x derive API

[bpowers]

Replace the flat Args struct and manual parse_args() function with
clap's derive-based subcommand parsing. Each subcommand now declares
exactly the arguments it accepts, making the CLI self-documenting for
both humans and AI agents.

Key changes:

• Subcommands (simulate, convert, equations, debug, gen-stdlib, vdf-dump)
  are now a proper enum with per-variant fields
• Shared model-input args (path + format) use #[command(flatten)]
• Input format auto-detected from file extension (.mdl, .pb/.bin)
  with explicit --format override
• Mutually exclusive --to-xmile/--to-mdl replaced by --to enum
• --reference on debug is enforced required at parse time by clap
• New open_model() helper centralizes format-dispatch logic

Also fixes lint-project.sh to use a temp file instead of process
substitution, which requires /dev/fd and fails in some environments.

https://claude.ai/code/session_017qMQ634tovdKLeQmYgQMAm

[claude]

Code Review

Reviewed the migration from pico-args to clap 4.x derive API, the open_model() helper, resolve_input_format(), and the lint-project.sh fix.

No bugs found. The refactoring is a clean 1:1 behavioral migration with some nice improvements:

• Subcommand-scoped arguments prevent invalid flag combinations at parse time
• --reference on debug is now enforced by clap instead of a manual runtime check
• Auto-detection of input format from file extension is a good UX addition
• The lint-project.sh temp file fix correctly avoids the /dev/fd process substitution issue

Overall correctness: Correct. The patch preserves existing behavior, the argument structure maps cleanly to the old flags, and no existing code paths are broken by the migration.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.00%. Comparing base (0fa352a) to head (37f1fde).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/simlin-cli/src/main.rs	0.00%	77 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #395      +/-   ##
==========================================
+ Coverage   76.92%   77.00%   +0.08%     
==========================================
  Files         143      143              
  Lines       36549    36511      -38     
==========================================
+ Hits        28115    28116       +1     
+ Misses       8434     8395      -39     

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[bpowers]

@codex review

[chatgpt-codex-connector]

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