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.

docs/config-catalog.md

@@ -1449,6 +1449,8 @@ Requires: `tools` · `fs` · `systemPrompt`
 ```ts config-catalog
 /** Plugin config (all optional — `Config` supplies the defaults). */
 export interface Config {
+  /** Maximum entries one `list` call renders inline; the footer still reports the complete count. */
+  listMaxEntries?: number
   /** Default and maximum number of lines returned by one `read` call. */
   readLimit?: number
   /** Maximum characters returned for a single line before truncation. */
@@ -1460,7 +1462,7 @@ export interface Config {
 }
 ```
 
-Source: [`packages/fs/tool-fs/src/index.ts:24`](../packages/fs/tool-fs/src/index.ts)
+Source: [`packages/fs/tool-fs/src/index.ts:27`](../packages/fs/tool-fs/src/index.ts)
 
 ## `@deepseek-ai/dsh-tool-fs-search`