feat!: require Monarch session cookie auth

[eshaffer321]

Summary

Monarch no longer accepts the legacy bearer token, so MONARCH_TOKEN stopped working. This switches itemize to cookie-based auth.

• Bump monarch-go to v2.1.0 (adds NewClientWithCookie)
• MONARCH_COOKIE / monarch.cookie is now required; MONARCH_TOKEN / monarch.api_key are removed
• Startup fails fast with a clear error when the cookie is missing (instead of failing on the first Monarch call)
• README, AGENTS.md, config.yaml, and CLI help updated with instructions for copying the cookie from DevTools

Breaking change

Users must replace MONARCH_TOKEN with MONARCH_COOKIE (sessionid=...; csrftoken=...). Release as v0.3.0.

Test plan

• New tests for newMonarchClient (config cookie, env fallback, missing-cookie error)
• Config tests updated for Cookie
• go test ./... -race passes

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 64.98%. Comparing base (c2a737a) to head (8bb29d6).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/adapters/clients/clients.go	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
==========================================
+ Coverage   64.95%   64.98%   +0.03%     
==========================================
  Files          51       51              
  Lines        6979     6983       +4     
==========================================
+ Hits         4533     4538       +5     
+ Misses       2108     2107       -1     
  Partials      338      338              

Flag	Coverage Δ
unittests	64.98% <85.71%> (+0.03%)	⬆️

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

Files with missing lines	Coverage Δ
internal/infrastructure/config/config.go	81.08% <100.00%> (ø)
internal/adapters/clients/clients.go	76.27% <83.33%> (+3.54%)	⬆️

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