feat(terminal): nano editor builtin (#2547)

## What changed
- New `nano` builtin in terminal sessions: a subset GNU-nano-style
editor with typing, cursor keys, `^O` write out, `^X` exit (asks to save
when modified), `^K`/`^U` cut and paste a line, `^W` search, `^G` help
and `^C` cursor position.
- `EDITOR=nano` works for the `$EDITOR` flows (`git commit` without
`-m`).
- The unsupported surface is documented as L-TERM-005; the threat model
entry TM-DOS-119 now covers nano's buffers.

## Why
Agents and the scripts they run often default to `nano`. Without it,
`EDITOR=nano git commit` failed and agents had to know to switch to vi.

## Before / After
Before: `nano notes.txt` → `nano: command not found` (127).
After: `nano notes.txt` → editor with title bar and shortcut bar; type,
`^O` Enter saves, `^X` returns to the prompt; `cat notes.txt` shows the
edit. `EDITOR=nano git commit` → type the message, `^X`, `y` → commit
created.

## Risk
- Low. `terminal` feature only (off by default); new builtin, no change
to existing commands.

## Checklist
- [x] Tests added or updated
- [x] Backward compatibility considered

---
_Generated by [Claude
Code](https://claude.ai/code/session_01X1g3pvv3cZECEhMrysGgJw)_

Author: chaliy

.github/workflows/ci.yml

@@ -324,7 +324,7 @@ jobs:
       # so the main "Run tests" step compiles them out entirely.
       - name: Run terminal tests
         run: |
-          cargo test -p bashkit --features terminal,git --lib -- terminal:: builtins::vi:: builtins::pager::
+          cargo test -p bashkit --features terminal,git --lib -- terminal:: builtins::vi:: builtins::pager:: builtins::nano::
           cargo test -p bashkit --features terminal --test integration -- terminal_tests l_term_
           cargo test -p bashkit --features terminal --doc terminal
 

crates/bashkit/src/builtins/mod.rs

@@ -83,6 +83,8 @@ mod ls;
 mod man;
 mod mapfile;
 mod mkfifo;
+#[cfg(feature = "terminal")]
+mod nano;
 mod navigation;
 mod nl;
 mod numfmt;
@@ -233,6 +235,8 @@ pub use tsort::Tsort;
 
 pub use jobctl::{Bg, Disown, Fg, Jobs, Kill, Pgrep, Ps};
 #[cfg(feature = "terminal")]
+pub use nano::Nano;
+#[cfg(feature = "terminal")]
 pub use pager::More;
 pub use sleep::Sleep;
 pub use sortuniq::{Sort, Uniq};