Respect launch environment for spawned commands

[TomCC7]

Summary

• Capture the launch environment once and reuse it when spawning git, tmux, and OpenCode commands.
• Forward launch env vars into tmux-created sessions with explicit -e KEY=value arguments.
• Add regression coverage, including a live tmux test for GIT_LFS_SKIP_SMUDGE propagation.

Verification

• cargo build
• cargo test --lib --quiet
• GIT_LFS_SKIP_SMUDGE=1 cargo test --lib tmux::tests::test_tmux_create_session_forwards_launch_env --quiet

Notes

• cargo clippy -- -D warnings still reports unrelated pre-existing warnings in src/app/dialogs.rs.