feat(computer-use): background focus-safe backend — set_value, structured windows, MIME detection

Extends the cua-driver computer-use backend to drive backgrounded macOS
windows without stealing keyboard or mouse focus from the foreground app.
All changes target the cua-driver MCP backend and the shared dispatcher.

## cua_backend.py

**Window-aware capture**: capture() now calls list_windows + get_window_state
instead of the removed capture tool. Prefers structuredContent.windows
(MCP 2024-11-05+ cua-driver) for zero-parse window enumeration; falls back
to regex-parsed text for older builds. Stores the selected (pid, window_id)
as sticky context so subsequent action calls do not need a redundant round-trip.

**Action routing**: click/scroll/type_text/key all carry the sticky pid
(and window_id for element-indexed clicks). type_text routes through
type_text_chars (individual key events) rather than AX attribute write --
WebKit AXTextFields reject attribute writes from backgrounded processes.

**Key parsing**: _parse_key_combo splits cmd+s-style strings into
(key, [modifiers]) and routes to hotkey (modifier present) or
press_key (bare key) -- cua-driver actual tool names.

**set_value method**: new set_value(value, element) calls the cua-driver
set_value MCP tool. For AXPopUpButton / HTML select in a backgrounded Safari,
AXPress opens the native macOS popup which closes immediately when the app is
non-frontmost; set_value AX-presses the matching child option directly
(no menu required, no focus steal).

**focus_app**: reimplemented as a pure window-selector (enumerates
list_windows, sets sticky pid/window_id) without ever raising the window
or stealing focus.

**list_apps**: fixed tool name from listApps to list_apps; handles plain-text
response via regex when structured data is absent.

**Structured-content extraction**: _extract_tool_result now surfaces
structuredContent from MCP results, enabling the list_windows window array
without text parsing.

**Helpers**: _parse_windows_from_text, _parse_elements_from_tree,
_split_tree_text, _parse_key_combo extracted as module-level functions.

## schema.py

Added set_value to the action enum with a description explaining when to
prefer it over click (select/popup elements, sliders, no focus steal).
Added value field for set_value payloads.

## tool.py

Routed set_value action through _dispatch to backend.set_value.
Added set_value to _DESTRUCTIVE_ACTIONS (approval-gated).
Fixed MIME-type detection in _capture_response: cua-driver may return
JPEG; detect from base64 magic bytes (/9j/ -> image/jpeg, else image/png)
rather than hardcoding image/png.

## agent/display.py + run_agent.py

Guard _detect_tool_failure and result-preview logic against non-string
function_result values: multimodal tool results (dicts with _multimodal=True)
are not string-sliceable; treat them as successes and fall back to str()
for length/preview.

tools/computer_use/schema.py

@@ -40,6 +40,7 @@ COMPUTER_USE_SCHEMA: Dict[str, Any] = {
                     "scroll",
                     "type",
                     "key",
+                    "set_value",
                     "wait",
                     "list_apps",
                     "focus_app",
@@ -47,7 +48,9 @@ COMPUTER_USE_SCHEMA: Dict[str, Any] = {
                 "description": (
                     "Which action to perform. `capture` is free (no side "
                     "effects). All other actions require approval unless "
-                    "auto-approved."
+                    "auto-approved. Use `set_value` for select/popup elements "
+                    "and sliders — it selects the matching option directly "
+                    "without opening the native menu (no focus steal)."
                 ),
             },
             # ── capture ────────────────────────────────────────────
@@ -132,6 +135,16 @@ COMPUTER_USE_SCHEMA: Dict[str, Any] = {
                 "type": "integer",
                 "description": "Scroll wheel ticks. Default 3.",
             },
+            # ── set_value ──────────────────────────────────────────
+            "value": {
+                "type": "string",
+                "description": (
+                    "For action='set_value': the value to set on the element. "
+                    "For AXPopUpButton / select dropdowns, pass the option's "
+                    "display label (e.g. 'Blue'). For sliders and other "
+                    "AXValue-settable elements, pass the numeric or string value."
+                ),
+            },
             # ── type / key / wait ──────────────────────────────────
             "text": {
                 "type": "string",