feat(ssh): support BREV_SSH_CONFIG_FILE env var override

[ishandhanani]

Summary

• Add BREV_SSH_CONFIG_FILE environment variable support to override the default ~/.ssh/config path
• When set, all brev SSH operations (refresh, etc.) edit the specified file instead
• Useful for nix-managed systems where the main SSH config is read-only

Test plan

• unset BREV_SSH_CONFIG_FILE && brev refresh -- should still edit ~/.ssh/config (default behavior)
• export BREV_SSH_CONFIG_FILE=~/.ssh/config.local && brev refresh -- should edit ~/.ssh/config.local instead

[Copilot]

Pull request overview

This PR adds support for overriding the SSH config file path via the BREV_SSH_CONFIG_FILE environment variable. This is useful for systems with read-only main SSH config files (like nix-managed systems), allowing users to specify an alternative location for brev's SSH configuration changes.

Changes:

• Added environment variable check in GetUserSSHConfigPath() to allow overriding the default ~/.ssh/config path
• The override is checked first before falling back to the default behavior

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 12.63%. Comparing base (272bf99) to head (1444d00).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/files/files.go	0.00%	15 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #288      +/-   ##
==========================================
- Coverage   13.10%   12.63%   -0.47%     
==========================================
  Files         112      115       +3     
  Lines       14480    15018     +538     
==========================================
  Hits         1898     1898              
- Misses      12349    12886     +537     
- Partials      233      234       +1     

Flag	Coverage Δ
Linux	12.63% <0.00%> (-0.47%)	⬇️

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

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.