fix(response-viewer): role on full-context blocks, divider ReDoS, pi mode (#326 follow-up)

Three post-merge fixes for the external-CLI response viewer:

- ?context=full blocks now carry role ('user' for prompts, 'assistant'
  for response/status/tool). The frontend's loadFullContext() renders
  via msg.role, so the roleless blocks lost the "You" badge and every
  turn rendered as the agent. kind/label/text are unchanged and the
  frontend needs no change.

- normalizeDividerStatusLine() dropped its backtracking regex
  (/^[─-]+\s*(.+?)\s*[─-]{3,}$/): the lazy middle went catastrophic on
  a long dash run without a 3-dash tail (measured 15.5s at 4,000 chars,
  minutes at 10,000), and pane text is agent-controlled with buffers up
  to 32MB. Replaced by a linear counter walk with the identical accept
  set and captured content, pinned char-for-char against the old regex
  by a brute-force corpus test plus a hostile-input regression test
  that fails by timeout with the RegExp version (same approach as the
  glob-matcher hardening in 68ae9a8).

- 'pi' joins EXTERNAL_CLI_MODES: pi sessions had the identical
  empty-viewer symptom the transcript branch exists to fix. The list
  stays a local duplicate of isExternalCliMode() (importing session.ts
  would drag node-pty into the pure module); a new exhaustive parity
  test asserts the two mode sets can no longer drift.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/routes/session-routes.ts

@@ -1903,11 +1903,11 @@ export function registerSessionRoutes(
       return await readCodexLastResponse(session, codexQuery.context === 'full');
     }
 
-    // OpenCode / Gemini / Antigravity render their own TUIs and write no Claude
-    // transcript, so the scan below finds nothing and the response viewer renders
-    // permanently empty for them. Segment the terminal buffer instead — the pane
-    // IS the transcript for these CLIs. Codex is already handled above, where a
-    // real rollout file is the better source.
+    // OpenCode / Gemini / Antigravity / Pi render their own TUIs and write no
+    // Claude transcript, so the scan below finds nothing and the response viewer
+    // renders permanently empty for them. Segment the terminal buffer instead —
+    // the pane IS the transcript for these CLIs. Codex is already handled above,
+    // where a real rollout file is the better source.
     if (isExternalCliTranscriptMode(session.mode)) {
       const externalQuery = req.query as { context?: string };
       const blocks = parseExternalCliTranscript(session.terminalBuffer, session.mode);