feat(desktop): polish agent-session transcript UI atop declared-ownership model

Replay of PR #1061 (tho/activity-ui-polish) onto fresh main: lands the
20 UI-polish commits as one net diff, reconciled with #1229's merged
declared-ownership model (viewerIsOwner = isCurrentUserOwner || isOwner)
and #1089's content-visibility-auto virtualization.

Notable reconciles:
- markdown.tsx: ported the compact/tight variant system into main's
  newer lightbox/spoiler markdown component (rather than overwriting it),
  layering variant density/leading overrides after the base owl-spacing
  so tailwind-merge wins. Dropped the branch's hardcoded text-[15px] in
  favor of main's rem-token text-sm base (post-#1052 zoom-safe scale).
- agentSessionTranscript.ts: pass TranscriptItemContext (not channelId).
- managed_agents: thread avatar_url through ManagedAgentSummary so the
  transcript renders the assistant-bubble avatar from the pinned record
  snapshot; bumped runtime.rs size override 2001 -> 2002 for the +1 line.

Observer-seed screenshots intentionally excluded (separate follow-up).

Co-authored-by: Taylor Ho <taylorkmho@gmail.com>
Signed-off-by: Taylor Ho <taylorkmho@gmail.com>

desktop/src/features/agents/ui/agentSessionTranscriptHelpers.test.mjs

@@ -67,6 +67,37 @@ test("parsePromptText extracts content, hex pubkey, and a title-cased kind", ()
   );
 });
 
+test("parsePromptText preserves multiline event content in the user bubble text", () => {
+  const text = [
+    "[Buzz event: @mention]",
+    "Event ID: event-1",
+    "Channel: agents",
+    `From: tho (hex: ${HEX})`,
+    "Time: 2026-06-15T17:15:00Z",
+    "Content: @Ned",
+    "",
+    "- remove that stray cherry pick if it's not adding value here",
+    "- help me understand what that e2eBridge change does",
+    "- we'd want the e2e seed path as a separate pull request",
+    'Tags: [["h","agents"]]',
+    "Parsed: mentions=[Ned]",
+  ].join("\n");
+
+  const result = parsePromptText(text);
+
+  assert.equal(
+    result.userText,
+    [
+      "@Ned",
+      "",
+      "- remove that stray cherry pick if it's not adding value here",
+      "- help me understand what that e2eBridge change does",
+      "- we'd want the e2e seed path as a separate pull request",
+    ].join("\n"),
+  );
+  assert.equal(result.userPubkey, HEX);
+});
+
 test("parsePromptText lowercases the extracted hex pubkey", () => {
   const text = [
     "[Buzz event: dm]",