fix: implement dynamic PTY resize (ResizePty)

[miguelgila]

Summary

• Implements file-based IPC between shim and runtime daemon for PTY resize
• Shim writes width height to /run/reaper/<id>/resize (or exec-<exec_id>-resize for exec sessions)
• Runtime daemon polls the resize file every 100ms and applies TIOCSWINSZ ioctl to the PTY master fd
• Supports both do_start() and exec_with_pty() PTY sessions
• All Linux-specific code properly gated with #[cfg(target_os = "linux")]

Closes #56

Test plan

• cargo clippy --workspace --all-targets -- -D warnings passes (macOS)
• cargo clippy --target x86_64-unknown-linux-gnu --all-targets -- -D warnings passes (Linux cross-compile)
• cargo test --workspace passes (152 tests)
• Manual verification: kubectl exec -it <reaper-pod> -- bash and resize terminal window, then run ps auxf to confirm full-width output

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.78%. Comparing base (b890734) to head (9e1ac78).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   85.01%   84.78%   -0.23%     
==========================================
  Files           6        6              
  Lines         307      309       +2     
==========================================
+ Hits          261      262       +1     
- Misses         46       47       +1     

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