Add rename secret support (press R)

[Copilot]

Secrets could be listed, added, edited, and deleted, but not renamed without a delete-then-add workaround. This adds first-class rename support.

Changes

• Tool.cs — New RenameSecret method: prompts key selection, then an editable name field pre-filled with the current name. Blocks rename if the target name already exists (no silent overwrites). Wired to R key in HandleInput; help line updated accordingly.
• ToolTests.cs — Unit tests covering: successful rename, collision prevention, and Escape-to-cancel.
• ConsoleUIIntegrationTests.cs — Integration tests verifying filesystem persistence for both the happy path and the collision guard.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Make it possible to rename secrets</issue_title>
<issue_description>Some times you want to rename a secret, either due to a new name being needed or just to easily switch between two different values (ie using a local or remote connection string).

This should be added as an option by pressing "r", then selecting the key to rename and then selecting a new name. If selecting an already used name it should not allow to overwrite the existing value.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Make it possible to rename secrets #91

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.78%. Comparing base (213d52f) to head (188e896).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #92      +/-   ##
===========================================
+ Coverage    69.92%   70.78%   +0.86%     
===========================================
  Files           10       10              
  Lines          542      558      +16     
  Branches        91       94       +3     
===========================================
+ Hits           379      395      +16     
  Misses         115      115              
  Partials        48       48              

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