a chat can change model mid-conversation

the selector locked on `hasStarted`, so the tier was fixed for the life of a conversation. nothing
justified it — the commit that added the flag says only "default model" — and the hazard it resembles is
narrower than what it blocked: crossing HARNESS hands an opencode `ses_…` id to `claude --resume`, which
reads as a dead agent rather than a wrong dispatch. changing TIER was always safe.

so tier is free, harness is pinned, and the server refuses a crossing rather than relying on the UI to.

the claude side does it the way the CLI's /model does: `Query.setModel()`, a control request that needs
streaming input mode, which is the only mode this file uses. nothing is torn down and nothing is
re-resumed — cwd, permission mode and transcript all continue and only the next response changes. an
earlier plan here was to kill the session and re-create it with the stored resume id; that would have
worked and was unnecessary.

opencode already switched per turn (serve-runner POSTs the model to the live session), so only the UI was
in its way.

FIXES A SILENT MISMATCH while it is here. model reached the SDK once, in createSession's query() options,
and every later turn was pushed onto that same call — so a changed model was dropped while session:init,
the cost fold and the durable log all reported the new tier. reachable today by leaving a chat idle past
the 30-minute GC: the UI said sonnet, opus answered, nothing logged it. the fold now tracks what the
harness actually did rather than what the client asked for, and a refused switch reverts the picker.

the switch is a durable `model:switched` event and a centred marker in the transcript, because the useful
question is the BOUNDARY — which half of this conversation is opus. per-message model was already stamped
server-side and shown nowhere, and claude-sessions.ts reads the FIRST model in a transcript, so a switch
was invisible in history.

tests: turn-stream proves the marker is durable and lands after the previous model's last words; the
selector tests pull in opposite directions, so unlocking everything fails one and restoring the blanket
lock fails the other. verified by re-breaking both.

not done: opencode's `steer` path (a send mid-turn) returns before the model POST, so a switch there
applies on the next full turn. session.model also stays stale for opencode tier changes — harness routing
is unaffected.

src/servers/sidecar/claude/turn-stream.test.ts

@@ -170,6 +170,33 @@ describe('createTurnStream', () => {
     ]);
   });
 
+  // A tier switch has to survive a reload, and it has to land in the right PLACE. The marker says "opus
+  // wrote everything above this line", which is false if it is emitted before the last thing opus said is
+  // flushed out of the delta buffer.
+  test('a model switch is durable and lands after the previous model’s last words', () => {
+    const { durable } = run([
+      { type: 'delta', text: 'the old model speaking' },
+      { type: 'model:switched', from: 'claude-code/opus', to: 'claude-code/sonnet', ok: true },
+      { type: 'text', text: 'the new one' },
+    ]);
+    expect(durable).toEqual([
+      { type: 'assistant:text', text: 'the old model speaking' },
+      { type: 'model:switched', from: 'claude-code/opus', to: 'claude-code/sonnet', ok: true },
+      { type: 'assistant:text', text: 'the new one' },
+    ]);
+  });
+
+  test('a refused switch is recorded too, with its reason', () => {
+    // Silence here would be the worst outcome: the turn answers as the OLD model while the picker shows the
+    // new one, which is the exact mismatch this whole change exists to remove.
+    const { durable } = run([
+      { type: 'model:switched', from: 'claude-code/opus', to: 'claude-code/haiku', ok: false, error: 'nope' },
+    ]);
+    expect(durable).toEqual([
+      { type: 'model:switched', from: 'claude-code/opus', to: 'claude-code/haiku', ok: false, error: 'nope' },
+    ]);
+  });
+
   test('streams are independent', () => {
     const a = createTurnStream('a');
     const b = createTurnStream('b');