feat: render chat summaries as a headline plus bullets - #28013

The whole-chat summary shown in the Summary tab rendered as a single
prose blob. Three layers each independently prevented structure: the
generation prompt banned all markdown, the normalizer collapsed every
newline into a space, and the panel rendered the result into one plain
<p>. Validation was also looser than the prompt asked, permitting up
to 1000 runes and six sentences in a 448px column.

Generation now returns a structured headline plus 2-4 bullets, which a
shared serializer renders to markdown stored in the existing summary
column. No migration is needed.

Both summary producers go through formatChatSummaryMarkdown so the
subagent path cannot bypass the format. Subagent summaries are
extracted from an existing report rather than generated, so they pass
no bullets and the serializer returns their snippet unchanged.

Validation moved onto the struct, before serialization. Applying the
old sentence cap to serialized markdown would have silently stopped
working, since bullets routinely omit trailing punctuation.
Per-field normalization also preserves backticks, which the shared
normalizeShortTextOutput strips, so a headline ending in an inline
code span keeps a balanced pair.

The panel renders the markdown through InlineMarkdown with a narrow
allowlist, bounded to max-h-48 with a Show more toggle. Legacy prose
summaries continue to render; summaries regenerate after three
completed turns, so both shapes coexist without a backfill.

Observe overflow on an inner unclamped element rather than the clamped
box. The clamped box stops growing at its max height, so a summary that
arrives via an in-place cache update while the box is already pinned
there resized nothing and stayed clipped with no toggle.

Drop useCallback, which site/AGENTS.md prohibits under
src/pages/AgentsPage since React Compiler already memoizes callbacks
there. The measurement closure now lives inside the effect.

Render link text without an anchor. Clipping below the collapsed bound
is visual only, so a mounted anchor stayed reachable by keyboard and
screen readers while invisible.

generateChatSummary now returns a headline plus bullets, but its doc
comment still described a 1-3 sentence summary. The subagent snippet
bound in chatd.go referenced the same obsolete contract.

…ummaries

Summaries preserve identifiers verbatim and wrap them in backticks, so a
single token can be wider than the panel with no natural break
opportunity. The summary box sized itself to that token, escaping its
column, and the clipped remainder was unreachable because the toggle
only responds to vertical overflow.

…rom summary story

FE10 prohibits DOM-geometry assertions because pixel values vary with
font rendering across environments. The long-identifier story keeps its
narrow column as a visual regression fixture and asserts semantically
instead.

…surement

Dragging the right panel resized the summary every frame, and each
observer callback read layout and set state even though the overflow
verdict is a boolean that flips at most a couple of times per drag.
Waiting for the resize to settle skips those reads entirely while a drag
is in flight.

…r chat

Switching chats swaps the panel's props instead of remounting it. An
uncached chat happened to reset the expanded state, because the panel
renders nothing while loading and that unmounts the summary, but a cached
chat resolves synchronously and kept the previous chat's expanded state,
offering to collapse a summary that may not overflow at all.

… disclosure toggle

The summary panel is already a scroll container, so clamping the summary
to a fixed height put a second, worse overflow mechanism in front of
content the reader could already reach by scrolling. Generated summaries
are capped at 600 runes server-side and rarely reach the clamp, so the
toggle mostly never appeared.

Removing it also removes the overflow measurement it required: the
ResizeObserver, the settle debounce, the two refs, and the per-chat key
that existed only to reset the expanded state.

…s dead

Normalization runs before validation and serialization, so several
downstream defensive checks could never fire:

- normalizeSummaryField guarded the empty case that strings.Fields
  already handles.
- formatChatSummaryMarkdown re-filtered blank bullets that
  normalizeSummaryBullets had already dropped, and built the output
  with a Builder plus a trailing-newline trim where a Join does.
- validateGeneratedChatSummary rejected bullets containing newlines,
  which strings.Fields collapses before validation sees them.

Record the normalization precondition on formatChatSummaryMarkdown so
the invariant is documented rather than re-checked, and drop the tests
that only covered the dead branches. Blank-bullet and newline handling
stay covered by TestNormalizeSummaryBullets and TestNormalizeSummaryField
at the layer that actually performs them.

Also drop the duplicated LongSummary panel story; it repeated the
ChatSummary story verbatim while asserting nothing about the panel.

Merge origin/main, preserve both database and frontend changes, and renumber the summary migration to 000591.\n\nGenerated by Coder Agents.