A directory the model knows about, and @ to name a file in it

An agent chat used to open with the model knowing the name of a machine and
nothing about what was on it, so the first two rounds of every reply went on
finding out. It now gets a listing: one read-only command, `git ls-files` where
that works and `find` otherwise, falling back to an SFTP walk that always does.
git first because a repository already carries somebody's considered list of
what is not part of the project, and reproducing it by hand is how an index
ends up mostly build output.

The listing is budgeted rather than dumped. A tree of a thousand files is worse
than no tree -- it costs the window on every request forever and buries the four
names that mattered -- so directories that will not fit are shown as a count and
the model is told to open one itself. Collapsing picks the deepest and largest
first: by saving alone it would take `src/` before `src/web/static/vendor/`,
because it contains it, and lose every name worth having.

Read from a cache and never fetched. `harness.context_variables` is synchronous
and sits on the request path; the walk happens in the generation setup, which is
async and already doing network work, with a short wait. A chat whose first
reply outruns its first walk simply has no listing that turn and the fragment
disappears rather than appearing as an empty heading.

Then `@`, over the same index and over the library, and `/` for commands with an
Alt-based keyboard for the same jobs. A mentioned file arrives as contents, not
a reference -- a small model asked to call file_read often does not bother -- and
it arrives with its absolute path and the machine it came from, because a model
handed `main.py` cannot tell which of four it is and cannot name it back when
asked to change something.

The rule that matters for `/`: a message that merely starts with a slash still
sends. `//` escapes and an unrecognised command is posted as written. Swallowing
somebody's message is a much worse failure than an unknown command.

Two exceptions to Manual mode now, not one. Browsing and indexing are a person
acting, not a model, so neither passes through policy.py -- the same argument
the terminal panel rests on.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/templates/chat/_usage.html

@@ -0,0 +1,80 @@
+{#
+  What `/usage` shows.
+
+  Two different numbers, deliberately kept apart. **Used** is how full the
+  window is right now -- one reply's prompt plus its completion, which is what
+  decides when compaction fires. **Spent** is everything this conversation has
+  cost end to end, which is larger and grows forever: a three-round reply pays
+  for its prompt three times but only ever occupies the window once.
+
+  Anything derived from an estimate wears a `~`, because `services/tokens.py`
+  counts four characters to a token when an endpoint reports nothing, and a
+  precise-looking figure that is a guess is worse than an obvious guess.
+#}
+<table class="sheet">
+  <tbody>
+    <tr>
+      <td>In the window now</td>
+      <td>
+        {% if metrics.context_tokens %}
+          {{ '~' if metrics.estimated }}{{ '{:,}'.format(metrics.context_tokens) }} tokens
+          {% if metrics.percent %}
+            — {{ metrics.percent }}% of
+            {{ '{:,}'.format(metrics.context_limit) }}
+          {% endif %}
+        {% else %}
+          Nothing yet.
+        {% endif %}
+      </td>
+    </tr>
+
+    {% if not metrics.context_limit %}
+    {# Unknown is not zero. Nobody has said how big this model's window is, so
+       the percentage is omitted rather than computed, and automatic
+       compaction never fires. #}
+    <tr>
+      <td>Window size</td>
+      <td>
+        Not recorded for {{ model.label if model else "this model" }}, so there is
+        no percentage and this chat will never compact itself.
+        {% if user.is_admin %}
+          Set it under <a href="/admin/models">Models</a>.
+        {% endif %}
+      </td>
+    </tr>
+    {% endif %}
+
+    <tr>
+      <td>Spent in total</td>
+      <td>
+        {{ '~' if estimated }}{{ '{:,}'.format(totals.total) }} tokens
+        across {{ replies }} repl{{ 'y' if replies == 1 else 'ies' }}
+      </td>
+    </tr>
+    <tr>
+      <td>Of which sent</td>
+      <td>{{ '~' if estimated }}{{ '{:,}'.format(totals.prompt) }} tokens</td>
+    </tr>
+    <tr>
+      <td>Of which written</td>
+      <td>{{ '~' if estimated }}{{ '{:,}'.format(totals.completion) }} tokens</td>
+    </tr>
+
+    {% if metrics.tokens_per_second %}
+    <tr>
+      <td>Last reply</td>
+      <td>{{ '%.1f'|format(metrics.tokens_per_second) }} tokens/second</td>
+    </tr>
+    {% endif %}
+
+    {% if chat.compact_summary %}
+    <tr>
+      <td>Compacted</td>
+      <td>
+        Earlier turns are summarised. They are still in the transcript; they
+        just stop being sent.
+      </td>
+    </tr>
+    {% endif %}
+  </tbody>
+</table>