fix(slack): populate thread session with existing thread history (#7610)

* feat(slack): populate thread session with existing thread history

When a new session is created for a Slack thread, fetch and inject
the full thread history as context. This preserves conversation
continuity so the bot knows what it previously said in the thread.

- Add resolveSlackThreadHistory() to fetch all thread messages
- Add ThreadHistoryBody to context payload
- Use thread history instead of just thread starter for new sessions

Fixes #4470

* chore: remove redundant comments

* fix: use threadContextNote in queue body

* fix(slack): address Greptile review feedback

- P0: Use thread session key (not base session key) for new-session check
  This ensures thread history is injected when the thread session is new,
  even if the base channel session already exists.

- P1: Fetch up to 200 messages and take the most recent N
  Slack API returns messages in chronological order (oldest first).
  Previously we took the first N, now we take the last N for relevant context.

- P1: Batch resolve user names with Promise.all
  Avoid N sequential API calls when resolving user names in thread history.

- P2: Include file-only messages in thread history
  Messages with attachments but no text are now included with a placeholder
  like '[attached: image.png, document.pdf]'.

- P2: Add documentation about intentional 200-message fetch limit
  Clarifies that we intentionally don't paginate; 200 covers most threads.

* style: add braces for curly lint rule

* feat(slack): add thread.initialHistoryLimit config option

Allow users to configure the maximum number of thread messages to fetch
when starting a new thread session. Defaults to 20. Set to 0 to disable
thread history fetching entirely.

This addresses the optional configuration request from #2608.

* chore: trigger CI

* fix(slack): ensure isNewSession=true on first thread turn

recordInboundSession() in prepare.ts creates the thread session entry
before session.ts reads the store, causing isNewSession to be false
on the very first user message in a thread. This prevented thread
context (history/starter) from being injected.

Add IsFirstThreadTurn flag to message context, set when
readSessionUpdatedAt() returns undefined for the thread session key.
session.ts uses this flag to force isNewSession=true.

* style: format prepare.ts for oxfmt

* fix: suppress InboundHistory/ThreadStarterBody when ThreadHistoryBody present (#13912)

When ThreadHistoryBody is fetched from the Slack API (conversations.replies),
it already contains pending messages and the thread starter. Passing both
InboundHistory and ThreadStarterBody alongside ThreadHistoryBody caused
duplicate content in the LLM context on new thread sessions.

Suppress InboundHistory and ThreadStarterBody when ThreadHistoryBody is
present, since it is a strict superset of both.

* remove verbose comment

* fix(slack): paginate thread history context fetch

* fix(slack): wire session file path options after main merge

---------

Co-authored-by: Peter Steinberger <steipete@gmail.com>

src/config/schema.hints.ts

@@ -337,6 +337,7 @@ const FIELD_LABELS: Record<string, string> = {
   "channels.slack.userTokenReadOnly": "Slack User Token Read Only",
   "channels.slack.thread.historyScope": "Slack Thread History Scope",
   "channels.slack.thread.inheritParent": "Slack Thread Parent Inheritance",
+  "channels.slack.thread.initialHistoryLimit": "Slack Thread Initial History Limit",
   "channels.mattermost.botToken": "Mattermost Bot Token",
   "channels.mattermost.baseUrl": "Mattermost Base URL",
   "channels.mattermost.chatmode": "Mattermost Chat Mode",
@@ -480,6 +481,8 @@ const FIELD_HELP: Record<string, string> = {
     'Scope for Slack thread history context ("thread" isolates per thread; "channel" reuses channel history).',
   "channels.slack.thread.inheritParent":
     "If true, Slack thread sessions inherit the parent channel transcript (default: false).",
+  "channels.slack.thread.initialHistoryLimit":
+    "Maximum number of existing Slack thread messages to fetch when starting a new thread session (default: 20, set to 0 to disable).",
   "channels.mattermost.botToken":
     "Bot token from Mattermost System Console -> Integrations -> Bot Accounts.",
   "channels.mattermost.baseUrl":