Files, open beside the conversation

A third side panel, built the way the terminal is and filled the way the
inspector is: tabs holding open files. Project files over SFTP in an agent
chat; notes, skills, knowledge documents, this chat's text attachments and its
own scratch document everywhere. Read with pygments, edited in a plain
textarea, saved with a conflict check.

A bug found on the way in, and the reason this needed its own read path.
`ssh.read_file` ends in `clean_output`, which strips ANSI escapes and decodes
with errors="replace" -- right for the output of a command, and fatal for an
editor: open a file containing an escape byte, press Save, and you have
silently rewritten it with the escapes gone and every undecodable byte replaced
by U+FFFD. `read_text`/`write_text` decode strictly, report binary rather than
mangling it, carry an mtime:size token for a file that moved underneath, and
refuse an oversize write rather than truncating -- `write_file` truncates
because a model is told how many bytes it wrote, and somebody pressing Save is
not. The model-facing pair is untouched: what it returns is a contract a model
has been shown. A truncated read opens read-only for the mirror-image reason.

Six sources go through one dispatch table, for the reason tool_labels.py is a
table: six independently written permission checks is how one ends up written
slightly differently, and that failure looks like editing somebody else's note.

A save on a project file bypasses agent/policy.py, which makes it the fourth
documented exception to "the modes do not govern the keyboard" and the first
that writes. Same argument as the terminal panel -- whoever owns the credential
could write the file with scp -- but the consequence is larger and is now said
out loud rather than left to be inferred.

The model opens tabs from the file tools it was already calling, so no new
schema and no tokens. It never brings one to the front: an agent reads forty
files in a long reply, and taking the screen each time would drag somebody
through all of them and lose any edit in progress. Only the strip is streamed,
guarded on truthiness so the frame can never blank itself -- an empty one would
close every open tab, the approval card you could press twice with the sign
reversed. Both halves are settled on the server, which is why canvas.js needs
no guard against a swap at all.

No vendored editor. CodeMirror 6 needs a bundler, which is hard rule 1;
CodeMirror 5 would be a larger payload than xterm on every page, and xterm is
the one heavy dependency precisely because it loads only where it can be used.
So: server-rendered highlighting for reading, a textarea for writing, and the
panel says there is no colour while you type rather than pretending.

Also here: a scratch document per chat, with `scratch_write` at RISK_READ on
plan_update's argument, and a test pinning the three numbers that decide a
panel's width -- LAYOUT_BOUNDS drops an unknown variable silently, so a panel
missing from it has a drag handle that works and forgets.

Driven under a DOM stub and against the running application.

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

PLAN.md

@@ -43,11 +43,19 @@ be a different project, not a refactor.
 - [x] **Stop** — the send button becomes Stop while writing; what arrived is kept
 - [x] **Rewind** — edit one of your own turns and the conversation runs on from
       there. Truncates rather than branching
-- [x] Copy, regenerate, automatic chat titles
+- [x] **Chat titles that fit the chat** — an ordinary chat is named by a model
+      from the first exchange, an agent chat from its opening words alone, which
+      are already an objective. Renameable from the heading and from the sidebar
+      row; one response updates both
 - [x] Chats created on first message, so an abandoned composer leaves nothing
 - [x] **Unread indicator** — a green dot and a toast when a reply lands while
       you were elsewhere
-- [x] Folders, arbitrarily nested; deleting one keeps the chats inside it
+- [x] **Folders that carry something** — arbitrarily nested, with a name, a
+      description, a system prompt inherited by the chats inside them, and seeds
+      for the model, the kind and the agent target. Deleting one keeps the chats
+- [x] **The sidebar splits Chat and Agent** — a switch below the pinned models,
+      stored on the account, filtering the folder tree as well as the loose
+      chats
 - [x] Per-reply metrics — tokens, context used as a percentage, tokens/second,
       live while streaming and kept afterwards. Estimated with a `~` when the
       endpoint reports no usage
@@ -55,6 +63,11 @@ be a different project, not a refactor.
       the model's context. Summarised turns are kept and collapsed, not deleted
 - [x] Temporary chats — never listed, swept after a day, with a Keep button
 - [x] An admin-only request inspector beside the thread
+- [x] **Canvas** — a third side panel holding open files, in tabs. Project files
+      over SFTP in an agent chat; notes, skills, knowledge documents, this
+      chat's text attachments and its own scratch document everywhere. Read with
+      syntax highlighting, edited in a plain textarea, saved with a conflict
+      check. Files the model touches open themselves, without taking the screen
 
 ### Tools
 - [x] **Tool calling** — one reply is a bounded loop of requests, not one
@@ -229,7 +242,10 @@ be a different project, not a refactor.
       `/api/`: a reply is an event stream and caching one breaks it
 - [x] Two themes (`moria`, `shire`) from one set of design tokens
 - [x] Every control sized from `--control-h`, so rows line up by construction
-- [x] Toasts and dialogs of our own; no `window.confirm` anywhere
+- [x] Toasts and dialogs of our own; no `window.confirm` anywhere, and
+      `data-prompt` for asking one line before a request goes out
+- [x] **An approval card's command can be corrected** before it is allowed, and
+      the transcript says who wrote what ran
 - [x] Original SVG artwork generated from a single source
 
 ### Operations