fix: keep SSH agent forwarding stable across reconnects (#506)

[MisterTea]

Summary

• Keep SSH agent forwarding pointed at a stable per-client symlink, and retarget that symlink to the current SSH_AUTH_SOCK after an in-process reconnect and on saved-session reattach.
• Returning clients still reuse the original server-side reverse-tunnel destination, so the remote SSH_AUTH_SOCK does not change when the local agent socket does.

Closes #506.

Test plan

• refreshAgentProxyPath creates and retargets agent proxy socket
• TerminalClient reattach retargets SSH agent proxy
• TerminalClient in-process reconnect retargets SSH agent proxy
• bash format.sh
• ctest --parallel --output-on-failure in the issue-506 worktree
• CI on this push

[codecov]

Codecov Report

❌ Patch coverage is 77.06422% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.88%. Comparing base (044bcb5) to head (0b2c80c).

Files with missing lines	Patch %	Lines
src/terminal/TerminalClient.cpp	58.20%	19 Missing and 9 partials ⚠️
test/unit_tests/TerminalClientTest.cpp	85.03%	9 Missing and 13 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #828    +/-   ##
========================================
  Coverage   79.88%   79.88%            
========================================
  Files         152      153     +1     
  Lines       22144    22354   +210     
  Branches    14338    14472   +134     
========================================
+ Hits        17690    17858   +168     
- Misses       2641     2661    +20     
- Partials     1813     1835    +22     

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