Remove non-functioning brev secret command

[theFong]

Summary

• Remove the brev secret CLI command which was a non-functioning feature
• Delete the secret store types and API method (pkg/store/secret.go)
• Remove command registration from the CLI root

Test plan

• Project compiles cleanly after removal
• Verify brev secret is no longer available in CLI help output

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 12.79%. Comparing base (272bf99) to head (fbd7962).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
- Coverage   13.10%   12.79%   -0.31%     
==========================================
  Files         112      113       +1     
  Lines       14480    14829     +349     
==========================================
  Hits         1898     1898              
- Misses      12349    12698     +349     
  Partials      233      233              

Flag	Coverage Δ
Linux	12.79% <ø> (-0.31%)	⬇️

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