repl + renderer: streaming assistant output (Phase 1)

repl.ask_ai now drives broker.chat_stream and pumps each delta into
renderer.assistant_delta(delta) as it arrives. renderer.assistant_flush
is called when the stream ends to add a trailing newline if missing.
The full reassembled response is then handed to executor.extract_cmd_lines
for the CMD: confirm-and-execute path (unchanged from Phase 0).

renderer.assistant() is kept for non-streaming callers (none in tree
right now, but cheap to keep around). assistant_delta/flush share no
state with assistant(); they use a module-local stream_buf that tracks
the in-progress streamed block.

Q12 deferred: incremental CMD: highlighting (cursor-positioning re-
render on flush) is not implemented in Phase 1 — deltas emit raw. The
§6 CMD: marker is still extractable on the reassembled string post-
stream, which is what executor cares about. Renderer's bold+cyan
treatment for CMD: lines stays available via M.assistant().

Broker error / SSE-framed api-error path still pops the user turn and
restores ctx.pending_exec_output. Order: assistant_flush always runs
(even on error) so the cursor lands on a fresh line before the broker-
error status renders.

Live verification: `Count one to ten` against hossenfelder fast streams
deltas through to stdout incrementally; CMD: extraction works on the
reassembled string; confirm gate intact.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

repl.lua

@@ -68,21 +68,30 @@ function M.run(config)
         end
     end
 
-    -- Send user text to the active model, render the response, and (per
-    -- §6 + config.shell.confirm_cmd) optionally execute extracted CMD: lines.
+    -- Send user text to the active model, render the response token-by-token
+    -- via broker.chat_stream, and (per §6 + config.shell.confirm_cmd) optionally
+    -- execute extracted CMD: lines on the reassembled full text.
     local function ask_ai(text)
         local prev_pending = ctx.pending_exec_output
         ctx:append_user(text)  -- flushes any pending [exec output] as prefix
-        local resp, err = broker.chat(active_cfg, ctx:to_messages())
-        if not resp then
+
+        local parts = {}
+        local ok, err = broker.chat_stream(active_cfg, ctx:to_messages(),
+            function(delta)
+                parts[#parts + 1] = delta
+                renderer.assistant_delta(delta)
+            end)
+        renderer.assistant_flush()
+
+        if not ok then
             renderer.status("broker error: " .. tostring(err))
             table.remove(ctx.turns)              -- back out the merged user turn
             ctx.pending_exec_output = prev_pending  -- restore buffered exec output
             return
         end
+        local resp = table.concat(parts)
         ctx:append({ role = "assistant", content = resp })
         status_evictions(ctx:enforce_budget())
-        renderer.assistant(resp)
 
         for _, cmd in ipairs(executor.extract_cmd_lines(resp)) do
             local doit