feat(builtins): add tree command (#581)

## Summary
- Add `tree` builtin for directory tree visualization
- Supports `-a` (hidden files), `-d` (dirs only), `-L N` (depth limit),
`-I pattern` (exclude)
- Unicode box-drawing characters, file/directory count summary
- 10 unit tests covering all options and edge cases

## Test plan
- [x] Unit tests for basic tree, hidden files, dirs-only, depth limit,
exclude pattern
- [x] Error cases: nonexistent dir, invalid depth, invalid option, empty
dir
- [x] `cargo clippy --all-targets --all-features -- -D warnings` passes
- [x] Full test suite passes (1621 tests)

Closes #538

Author: chaliy

crates/bashkit/src/builtins/mod.rs

@@ -65,6 +65,7 @@ mod system;
 mod test;
 mod textrev;
 mod timeout;
+mod tree;
 mod vars;
 mod wait;
 mod wc;
@@ -120,6 +121,7 @@ pub use system::{DEFAULT_HOSTNAME, DEFAULT_USERNAME, Hostname, Id, Uname, Whoami
 pub use test::{Bracket, Test};
 pub use textrev::{Rev, Tac};
 pub use timeout::Timeout;
+pub use tree::Tree;
 pub use vars::{Eval, Local, Readonly, Set, Shift, Shopt, Times, Unset};
 pub use wait::Wait;
 pub use wc::Wc;