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/acp-agent/tests/snapshots/workspace-context/system-prompt.expected.md

@@ -5,6 +5,8 @@ You are a coding assistant powered by the deepseek-v4-flash model. Your working
 Verify your work by running the code or tests. Keep answers brief and factual.
 
 
+Use the list tool — not shell ls — to see what a directory contains. It returns the direct children of one directory, files and subdirectories alike, and defaults to the session workspace, so it is the first step for orienting in an unfamiliar project. Reach for glob or grep once you know the path pattern or the text you are looking for.
+
 Use the read tool — not shell commands like cat — to inspect text files. Results include line numbers. Use offset and limit to continue reading large files.
 
 Use the write tool to create files or completely replace file contents. Existing files are overwritten, so read an existing file first (the default fs-policy requires it) and prefer edit for targeted changes.

examples/acp-agent/tests/snapshots/workspace-context/tool-schemas.expected.json

@@ -115,6 +115,19 @@
         "properties": {}
       }
     },
+    {
+      "name": "list",
+      "description": "List the direct children of one directory, with their type. Entries are directories first, then files, each alphabetical; the first 200 are returned inline and the footer reports the complete count. Unlike glob, this shows subdirectories, so it is how to see what a directory contains.",
+      "parameters": {
+        "type": "object",
+        "properties": {
+          "path": {
+            "type": "string",
+            "description": "Directory to list. Defaults to the session workspace; a relative path resolves against it."
+          }
+        }
+      }
+    },
     {
       "name": "ralph",
       "description": "Run a foreground fresh-agent Ralph loop toward one immutable objective. Use only when the direct human explicitly asks for Ralph or fresh-agent iteration. Each round opens a new child with no parent conversation or prior child session; the shared workspace is long-term memory, and only a bounded structured report crosses rounds. The call returns when a worker reports completion or a concrete blocker, or at the round limit. Ordinary long-running same-session work belongs to goal tools.",