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/fs-list/workspace/README.txt

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+# Project

examples/acp-agent/tests/snapshots/fs-list/workspace/docs/guide.txt

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+# Guide

examples/acp-agent/tests/snapshots/fs-list/workspace/package.json

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+{ "name": "demo" }

examples/acp-agent/tests/snapshots/fs-list/workspace/src/index.txt

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+export const answer = 42