A time in no particular zone, and a preview missing what it previews

The first audit pass: everything from 0.8.1 to 0.9.8 read as a whole rather
than one feature at a time, starting with what a model is actually told.

Four of these had shipped as correct. The date line carried a timezone
variable that resolves to nothing until somebody chooses one -- so every
default account was told times were "in  unless they say otherwise", while
two comments asserted the line disappeared instead. The prompt preview
built its variables without a chat, which is what eleven fragments are
gated on, so the whole agent surface was absent from it whatever was
ticked. Plan mode was instructed to keep its plan current with a tool that
mode withdraws. And knowledge_get returned a document whole where every
sibling reader caps and says so, its description promising exactly that.

The subagent guidance was wrong in both directions at once: it denied a
documented parameter and named seven of twenty-three allowed commands.
Both halves are pinned by tests against the real list and the real schema
now, because prose and a constant drift the moment one is edited alone.

docs/notes/audit-0.9.md carries the findings that are not fixed here, with
why -- the ones whose fix would change what a feature does are the user's
call, not this pass's.

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

src/lembas/services/subagent.py

@@ -516,14 +516,17 @@ def tool_defs() -> list[ToolDef]:
             family=FAMILY_SUBAGENT,
             description=(
                 "Hand one self-contained piece of work to a helper — a second "
-                "model with the same tools that works on its own and gives you "
-                "its answer. Use it to cover several independent areas at once: "
-                "call it several times in one turn and each runs in parallel. "
-                "It cannot ask you or the reader anything, cannot delegate "
-                "further, and starts knowing nothing about this conversation, "
-                "so the task must say everything it needs. Do not use it for "
-                "something you could do in one call yourself, or for anything "
-                "needing a decision only the reader can make."
+                "model that works on its own and gives you its answer. Use it "
+                "to cover several independent areas at once: call it several "
+                "times in one turn and each runs in parallel. It gets a "
+                "narrower set of tools than you: it reads, it cannot ask you or "
+                "the reader anything, it cannot delegate further, it changes "
+                "nothing unless you set write, and on a machine it may run only "
+                "a fixed list of read-only commands. It starts knowing nothing "
+                "about this conversation, so the task must say everything it "
+                "needs. Do not use it for something you could do in one call "
+                "yourself, or for anything needing a decision only the reader "
+                "can make."
             ),
             parameters={
                 "type": "object",