phase: 138_markdown_blockquotes

**Phase 138 final verification pass — all green, no defects found.**

- Verified: blockquote pass in `frontend/assets/markdown.js` (step 2b, between escape+inline and paragraphs), guard extended to `|blockquote`, docstring updated; `.md-quote` CSS (left border + padding + `--ink-soft`, no fill, palette vars only, contrast comments, user-bubble override); unit pins + node behavior; dedicated E2E (chat / viewer / restore); mock `QUOTE_TRIGGER`/`QUOTE_ANSWER`; fixture `tests/fixtures/quote_docs/`. Thinking block confirmed on the same shared renderer (app.js L2025/L3095).
- `uv run pytest tests/e2e/test_markdown_blockquotes.py -v --no-cov` → 3 passed (isolation)
- `uv run pytest --cov=app --cov-report=term-missing` → 3268 passed, 9 skipped; TOTAL **99%** (>90%)
- `uv run ruff check . && uv run pyright` → All checks passed; 0 errors
- Criteria: quotes render in chat/viewer/restore ✅ (E2E) + thinking ✅ (shared renderer + unscoped AA CSS); `>>` collapses, fence/table `>` untouched ✅ (unit+e2e); non-quote byte-identical ✅ (exact fixture; phase-44/renderer suites pass **unedited**, confirmed by empty `git diff --stat`)
- No deviations; commit + phase-dir move left to the harness per pipeline rules (working tree clean of stray changes — only phase-138 diffs present)
- Next pending phase: **139_read_range_tool_lines**

tests/e2e/mock_llm.py

@@ -382,6 +382,19 @@ Implements just enough of the aipi surface:
     ``SUMMARY_MODE``), so a marker question always gets the table
     answer; the E2E asks it against an on-topic fixture (HIGH gate) and
     asserts non-deflection.
+  - user message containing ``show me the blockquote`` (phase 138,
+    markdown blockquotes, TODO.md L3) -> the fixed quote answer
+    (``QUOTE_ANSWER``): a single-line quote, a 3-line quote group, a
+    nested ``>>`` collapse line, an ``<script>`` XSS probe quote, a
+    ``>`` inside a code fence, a ``>`` in a table cell, and a plain
+    closing line — byte-stable, so the story E2E can assert the four
+    rendered ``<blockquote class="md-quote">`` shapes, the inert XSS
+    line, the untouched fence and table-cell ``>``, and the restore
+    path's byte-identical re-render. Checked BEFORE the
+    ``DEFLECT_MODE`` branch (a deflection prompt never carries the
+    marker, same reasoning as ``SUMMARY_MODE``), so a marker question
+    always gets the quote answer; the E2E asks it against an on-topic
+    fixture (HIGH gate) and asserts non-deflection.
   - user message containing ``echo my history``
     (``HISTORY_TRIGGER``, phase 74, TODO L4 — chat history with prior
     thinking reaches the LLM) -> the deterministic HISTORY ECHO, derived
@@ -955,6 +968,43 @@ TABLE_ANSWER = (
     "| value-one | value-two | value-three | value-four | value-five |"
 )
 
+#: Phase 138 (markdown blockquotes, TODO.md L3): a user message
+#: containing this substring (case-insensitive) gets the fixed quote
+#: answer (``QUOTE_ANSWER`` below) — the renderer's whole blockquote
+#: contract in one byte-stable answer (see the module docstring).
+#: Existing E2E questions do not contain the phrase, so every other
+#: suite is unaffected.
+QUOTE_TRIGGER = "show me the blockquote"
+
+#: The fixed quote answer (phase 138) — byte-stable on purpose: the
+#: story E2E asserts the four rendered <blockquote class="md-quote">
+#: shapes (single line, 3-line group, nested collapse, XSS probe),
+#: the untouched fence line and table cell, and the plain closing
+#: line (the stream-completion marker — it lands last).
+QUOTE_ANSWER = (
+    "Here's the shape, in quotes:\n"
+    "\n"
+    "> a short single-line quote\n"
+    "\n"
+    "> the first line\n"
+    "> the second line\n"
+    "> the third line\n"
+    "\n"
+    ">> nested quotes collapse to one level\n"
+    "\n"
+    "> <script>alert(1)</script>\n"
+    "\n"
+    "```\n"
+    "> inside a fence stays fence content\n"
+    "```\n"
+    "\n"
+    "| Cell |\n"
+    "|---|\n"
+    "| > x stays a cell |\n"
+    "\n"
+    "And the quote ends here — a plain closing line."
+)
+
 # ---------------------------------------------------------------------------
 # Phase 67 (LLM retry, TODO.md L3): deterministic failure injection
 # ---------------------------------------------------------------------------
@@ -2720,6 +2770,16 @@ def compose_answer(body: dict[str, Any]) -> str:
         # against an on-topic fixture, where the gate is HIGH, and
         # asserts non-deflection as part of the table test.
         answer = TABLE_ANSWER
+    elif QUOTE_TRIGGER in user.lower():
+        # Markdown blockquotes (phase 138, TODO.md L3): the story E2E's
+        # deterministic quote answer — the renderer's blockquote
+        # contract in one byte-stable stream (single-line quote,
+        # 3-line group, nested collapse, the <script> XSS probe, the
+        # fence-protected >, the table-cell >, the plain close).
+        # Checked BEFORE the DEFLECT_MODE branch, like TABLE_TRIGGER:
+        # the marker lives in the user message, so a marker question
+        # always gets the quote answer whatever the gate says.
+        answer = QUOTE_ANSWER
     elif HISTORY_TRIGGER in user.lower():
         # Phase 74 (TODO L4, chat history): the deterministic history
         # echo — proves on the wire that the client's prior turns (and