feat(fs): add a model-facing directory listing tool

`ctx.fs.listDir` has shipped since the filesystem seam gained it, with skill
discovery as its only consumer; the model-facing tool was deferred to a
separate decision. Nothing else could answer "what is in this directory":
`rg --files` backs glob and grep and never emits a directory entry, so an
empty directory is invisible, no output says which names are directories,
and no output gives an entry count.

`list` takes an optional `path`, defaulting to the session workspace so the
common question needs no argument, and returns the direct children of one
directory with their type. Two presentation rules carry it: directories sort
first, then files, then non-regular children, each alphabetically — so
truncation loses leaves rather than the tree — and the footer always states
the complete listing's size and composition, so a capped view can never read
as a whole directory.

It emits no `fs/observed`: seeing a filename is not reading a file, and a
listing must never satisfy the read-before-write gate.

examples/tui-agent/tests/snapshots/code-mode-dispatch-spill/terminal.expected.txt

@@ -59,7 +59,7 @@ buffer
   style 1-1 inverse
 27| "────────────────────────────────────────────────────────────────────────────────────────────────────"
   style 0-99 dim
-28| "deepseek-v4-flash  /workspace/project  ↑123 ↓208  cache 99%  3% c"
+28| "deepseek-v4-flash  /workspace/project  ↑123 ↓208  cache 99%  4% c"
   style 0-93 dim
   style 96-99 dim
 29-35| <blank>