opentui(copy): mask chrome/gutters so free-form copy is clean

Audit selectable masking (free-code noSelect model) so a free-form drag over an
agent turn yields CLEAN pasteable content — no labels, summaries, carets, or
annotations. Newly masked (selectable={false}):
- toolPart: the whole collapsed header row (name + args-preview + duration +
  "(N lines)") summary; the "args"/"output" section labels; the args overflow
  "… +N more"; the "… omitted N" / "… +N more lines" truncation notes.
- messageLine: the streaming caret (▍) — a cursor glyph, not content.
- reasoningPart: the collapsible-section header label (Thinking/Thought + title).
- composer: the completion dropdown rows + the "Tab complete · Esc dismiss" hint.
Kept selectable (real content): assistant markdown, tool args values + output
body, user/system message text.

ui-tui-opentui-v2/src/view/composer.tsx

@@ -178,15 +178,20 @@ export function Composer(props: {
             paddingRight: 1
           }}
         >
+          {/* the completion dropdown is transient input chrome (menu rows + the
+              key-hint) — not transcript content — so it's excluded from mouse
+              selection (item 4). */}
           <For each={completions().slice(0, 8)}>
             {(c, i) => (
-              <text fg={i() === 0 ? theme().color.accent : theme().color.text}>
+              <text selectable={false} fg={i() === 0 ? theme().color.accent : theme().color.text}>
                 {c.display || c.text}
                 {c.meta ? `  ${c.meta}` : ''}
               </text>
             )}
           </For>
-          <text fg={theme().color.muted}>Tab complete · Esc dismiss</text>
+          <text selectable={false} fg={theme().color.muted}>
+            Tab complete · Esc dismiss
+          </text>
         </box>
       </Show>
       {/* prompt glyph + textarea — the glyph (item 3) marks the input line so the

ui-tui-opentui-v2/src/view/messageLine.tsx

@@ -61,7 +61,8 @@ export function MessageLine(props: { message: Message }) {
                 </text>
               }
             >
-              <text>
+              <text selectable={false}>
+                {/* streaming caret — a cursor glyph, not content (item 4) */}
                 <span style={{ fg: theme().color.muted }}>▍</span>
               </text>
             </Show>

ui-tui-opentui-v2/src/view/reasoningPart.tsx

@@ -50,7 +50,10 @@ export function ReasoningPart(props: { text: string; streaming?: boolean }) {
               <span style={{ fg: theme().color.accent }}>{expanded() ? '▼' : '▶'}</span>
             </text>
           </box>
-          <text>
+          {/* the header is a collapsible-section LABEL (Thinking/Thought + title)
+              — chrome, not the reasoning body — so a free-form drag yields only
+              the markdown body below, not the section label (item 4). */}
+          <text selectable={false}>
             {/* accent chevron marks it; muted label keeps reasoning in the dim,
                 secondary tier alongside tool calls (Ink hierarchy). */}
             <span style={{ fg: theme().color.muted }}>{label()}</span>

ui-tui-opentui-v2/src/view/toolPart.tsx

@@ -103,7 +103,11 @@ export function ToolPart(props: { part: ToolPartState }) {
           </text>
         </box>
         <box style={{ flexDirection: 'row', flexGrow: 1, minWidth: 0 }}>
-          <text>
+          {/* the whole header row is a collapsed SUMMARY (tool name + args-preview
+              + duration + "(N lines)") — chrome, not the copyable body — so a
+              free-form drag over a tool yields only the expanded output/args
+              content, never the header label (item 4). */}
+          <text selectable={false}>
             <span style={{ fg: theme().color.muted }}>{props.part.name}</span>
             <Show when={running()}>
               <span style={{ fg: theme().color.muted }}> …</span>
@@ -134,7 +138,8 @@ export function ToolPart(props: { part: ToolPartState }) {
         >
           <box style={{ flexDirection: 'column', flexGrow: 1, minWidth: 0 }}>
             <Show when={showArgs()}>
-              <text>
+              {/* section label — chrome, not content (item 4) */}
+              <text selectable={false}>
                 <span style={{ fg: theme().color.label }}>args</span>
               </text>
               {/* parsed key: value lines (tidy), or raw argsText when unparseable */}
@@ -158,14 +163,16 @@ export function ToolPart(props: { part: ToolPartState }) {
                   )}
                 </For>
                 <Show when={argEntries().length > ARGS_MAX}>
-                  <text>
+                  {/* overflow annotation — chrome, not content (item 4) */}
+                  <text selectable={false}>
                     <span style={{ fg: theme().color.accent }}>{`… +${argEntries().length - ARGS_MAX} more`}</span>
                   </text>
                 </Show>
               </Show>
             </Show>
             <Show when={showArgs() && hasOutput()}>
-              <text>
+              {/* section label — chrome, not content (item 4) */}
+              <text selectable={false}>
                 <span style={{ fg: theme().color.label }}>output</span>
               </text>
             </Show>
@@ -176,13 +183,15 @@ export function ToolPart(props: { part: ToolPartState }) {
                 </text>
               )}
             </For>
+            {/* truncation annotations — chrome (the "… omitted N" / "… +N more
+                lines" notes are not part of the real output body) (item 4). */}
             <Show when={props.part.omittedNote}>
-              <text>
+              <text selectable={false}>
                 <span style={{ fg: theme().color.muted }}>{`… omitted ${props.part.omittedNote}`}</span>
               </text>
             </Show>
             <Show when={body().hiddenLines > 0 && !props.part.omittedNote}>
-              <text>
+              <text selectable={false}>
                 <span style={{ fg: theme().color.accent }}>{`… +${body().hiddenLines} more lines`}</span>
               </text>
             </Show>