Warn users about affected workloads when updating or deleting secrets

[JAORMX]

Summary

When a secret is updated or deleted, users may not realize that MCP servers using that secret need to be restarted to pick up the change. This PR adds a warning message that lists any affected workloads.

• Add ListWorkloadsUsingSecret method to the Manager interface
• Implement the method by scanning all RunConfigs for secret references
• Update the secret set and secret delete commands to warn users
• Add unit tests for the new method

Example output after updating a secret:

Secret github-token set successfully

Warning: The following MCP servers use this secret and may need to be restarted:
  - github-mcp
  - code-review-mcp

Test plan

• Unit tests added for ListWorkloadsUsingSecret
• Lint passes
• Build passes
• Manual test: create a secret, run a server with it, update the secret, verify warning appears
• Manual test: delete a secret used by a server, verify warning appears

Fixes #201

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 29.16667% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.01%. Comparing base (fa9fa84) to head (bdf0383).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/workloads/manager.go	29.16%	13 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2945      +/-   ##
==========================================
- Coverage   56.02%   56.01%   -0.01%     
==========================================
  Files         328      328              
  Lines       32290    32314      +24     
==========================================
+ Hits        18090    18101      +11     
- Misses      12665    12668       +3     
- Partials     1535     1545      +10     

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