fix(macOS): start terminal sessions in home directory

[kaluli123123]

Summary

• start local shell sessions in HOME on macOS
• apply the same working directory to fallback, Bash, Zsh, and generic shell launches
• preserve the existing inherited-directory behavior on other platforms

Root cause

Local sessions left working_directory unset. The PTY therefore inherited the GUI application's process directory, which is not guaranteed to be the user's home directory on macOS.

Fixes #47.

Verification

• regression tests failed before the implementation and pass after it
• cargo +nightly fmt
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo deny check
• cargo test --workspace --all-features: 527 passed, 2 ignored
• cargo llvm-cov --workspace --all-features --fail-under-lines 80: all tests passed; repository line coverage is 67.79%, below the existing 80% gate

Notes

cargo deny reports the existing yanked transitive dependencies core2 0.4.0 and spin 0.9.8.

[kaluli123123]

@coderabbitai review

Please focus on the macOS-only working-directory boundary and cross-platform behavior.