feat: add chat summary tab in the right sidebar and per-chat cost endpoint - #26649

Add a reusable ChatSummary component surfaced via an info button placed to
the left of the chat sharing button in the chat header. The button opens a
"Summary" popover (400px on desktop, full width on mobile) showing the
chat's summary plus its Created/Updated dates and cumulative cost.

The summary text is the persisted whole-chat summary (chat.summary) from the
backend summary feature this branch is stacked on. It is generated
asynchronously and may be null until the first summary is produced, in which
case the popover renders a muted empty state.

Cost is served by a new per-chat endpoint, GET
/api/experimental/chats/{chat}/cost, that rolls up assistant-message cost
across a chat's root and child (subagent) chats and is authorized like the
other {chat} routes.

Resolve the actionable findings from the deep review of #26657:

- P0/CRF-2: renumber the chat summary migration from 000530 to 000534 to
  avoid colliding with 000530_relay_host_nats_port on main.
- CRF-3: scope summary_change/chat_summary_change merges to their own
  field so an equal-timestamp event cannot clobber the other summary;
  add the equal-timestamp regression tests.
- CRF-4/CRF-13: bail out of background summary generation when shutdown
  has begun so Close() is not blocked, and fix the stale comment.
- CRF-5: re-read the chat before the cadence gate so rapid turns do not
  both pass against a stale snapshot.
- CRF-7: add a CHECK constraint on chat_messages.cost_source.
- CRF-8: use a dedicated chatSummaryWriteTimeout for the summary write.
- CRF-17: only count sentence terminators at a word boundary so dotted
  identifiers do not inflate the count.
- CRF-1/CRF-6: deduplicate the usage-recording and model-override
  resolution paths shared by title and summary generation.
- CRF-15/CRF-16: cover ChatMessageVisibilityUser and model-only user
  messages in the summary tests.
- CRF-9/10/11/12/19/22: comment cleanups, errors.AsType, and drop the
  popover reference from the settings description.

CRF-23: add TestCountSentenceTerminators so the CRF-17 fix is covered.
Without it, removing the word-boundary guard in countSentenceTerminators
would not fail any test. The test asserts periods inside dotted
identifiers are not counted and that a dotted-identifier-dense summary
stays under summaryMaxSentences.

Resolve conflicts in coderd/x/chatd/chatd.go by routing both recordManualTitleUsage and recordChatSummaryUsage through the base branch's recordHiddenUsageMessageTx helper, and reimplementing that helper with this PR's atomic InsertChatAccountingMessage insert. This drops the removed UpdateChatMessageCostSource two-step tag and the last_model_config_id restore, which the dedicated accounting insert makes unnecessary.

Renumber the chat_history_ignore_accounting migration from 000531 to 000535 so it runs after 000534_chat_summary, which adds the cost_source column the recreated history triggers depend on.

…exclusion

Address review feedback on the chat history accounting fix:

- InsertChatAccountingMessage stores cost_source verbatim instead of NULLIF(@cost_source, ''), so an empty value fails the chat_messages CHECK instead of silently becoming NULL and re-enabling the history_version advance the trigger guard prevents.

- Document in coderd/x/chatd/ARCHITECTURE.md that hidden accounting rows tagged with cost_source do not advance history_version.

- Add a mock test for the recordChatSummaryUsage summary path and a DB assertion that an empty cost_source is rejected.

Move summary generation model overrides out of the persisted summary PR so the base feature always uses the chat model.

Co-authored-by: Cursor <cursoragent@cursor.com>

…feedback

Move the chat summary out of the top-bar info button + popover and into a
new left-most "Summary" tab in the right panel:
- Add ChatSummaryPanel, a data wrapper that renders the existing
  presentational ChatSummary inside the panel and fetches the chat + cost
  only while the tab is visible.
- Remove the info button, ChatSummaryPopover, and the ChatTopBar
  render-prop wiring. Summary is the default right-panel tab when no tab
  selection is persisted.

Address review feedback on the per-chat cost endpoint and UI:
- GetChatCostByChatID matches the chat family by indexed columns instead of
  COALESCE(root_chat_id, id) so Postgres can use the chat indexes.
- Drop the dead IsNotAuthorizedError branch (the middleware already returns
  404) and return a specific "Failed to get chat cost." 500, matching the
  sibling chat handlers.
- Surface chat and cost query errors and partial (unpriced) totals in the
  UI, with matching stories.
- Add ZeroMessages and ExcludesNonAssistantMessages query tests.
- Name the cost staleTime constant and clarify the SQL comments.

…-work

Resolve conflict in coderd/x/chatd/chatd.go: adopt the renamed summary threshold constants (summaryInitialTurnThreshold, summaryStaleTurnThreshold) from the base branch while keeping the subagentReportSummary* constants added on this branch.

Generated by Coder Agents.

Resolve merge conflicts in three files:

- coderd/x/chatd/quickgen.go: keep both "net/http" and "regexp"
  imports (both are used).
- coderd/exp_chats_internal_test.go: union the imports and keep all
  tests from both sides (TestGetChatCostSurfacesReadAuthzRace and
  TestGetChatCostQueriesRequestedChat from this branch, plus
  TestEnrichMissingChatAgentIDs from the base branch).
- coderd/x/chatd/turn_summary_internal_test.go: keep this branch's
  subagent-report summary persistence test
  (TestSuccessfulChildChatOutcomeStoresReportSummaryWithoutPush). Drop
  TestPendingChatPersistsSummaryButSkipsWebPush because the base branch
  removed the unused "pending"/"paused"/"completed" chat statuses
  (migration 000543, PR #27064); that status no longer exists.