feat: add chat debug retention purge (#24943)

> Mux is acting on Mike's behalf.

Adds configurable retention for chat debug data, including the purge
query, updated_at index, site config, experimental API, SDK types,
frontend lifecycle setting, and docs.

The purge deletes debug runs older than the configured retention window
and relies on existing cascades to delete steps. The default retention
is 30 days, and setting the value to 0 disables the purge.

coderd/database/queries.sql.go

@@ -2993,6 +2993,37 @@ func (q *sqlQuerier) DeleteChatDebugDataByChatID(ctx context.Context, arg Delete
 	return result.RowsAffected()
 }
 
+const deleteOldChatDebugRuns = `-- name: DeleteOldChatDebugRuns :execrows
+WITH deletable AS (
+    SELECT id, chat_id
+    FROM chat_debug_runs
+    WHERE updated_at < $1::timestamptz
+    ORDER BY updated_at ASC
+    LIMIT $2::int
+)
+DELETE FROM chat_debug_runs
+USING deletable
+WHERE chat_debug_runs.id = deletable.id
+    AND chat_debug_runs.chat_id = deletable.chat_id
+`
+
+type DeleteOldChatDebugRunsParams struct {
+	BeforeTime time.Time `db:"before_time" json:"before_time"`
+	LimitCount int32     `db:"limit_count" json:"limit_count"`
+}
+
+// updated_at is the retention clock, so the window starts after the run
+// stops being written to.
+// Intentionally no finished_at IS NOT NULL guard: abandoned in-flight rows
+// older than the cutoff are also purged.
+func (q *sqlQuerier) DeleteOldChatDebugRuns(ctx context.Context, arg DeleteOldChatDebugRunsParams) (int64, error) {
+	result, err := q.db.ExecContext(ctx, deleteOldChatDebugRuns, arg.BeforeTime, arg.LimitCount)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected()
+}
+
 const finalizeStaleChatDebugRows = `-- name: FinalizeStaleChatDebugRows :one
 WITH finalized_runs AS (
     UPDATE chat_debug_runs
@@ -3236,6 +3267,8 @@ type InsertChatDebugRunParams struct {
 	FinishedAt          sql.NullTime          `db:"finished_at" json:"finished_at"`
 }
 
+// updated_at is the retention clock used by DeleteOldChatDebugRuns.
+// Set it on every write to keep retention semantics correct.
 func (q *sqlQuerier) InsertChatDebugRun(ctx context.Context, arg InsertChatDebugRunParams) (ChatDebugRun, error) {
 	row := q.db.QueryRowContext(ctx, insertChatDebugRun,
 		arg.ChatID,
@@ -3503,6 +3536,7 @@ type UpdateChatDebugRunParams struct {
 // write-once-finalize pattern where fields are set at creation
 // or finalization and never cleared back to NULL. The @now
 // parameter keeps updated_at under the caller's clock.
+// updated_at is also the retention clock used by DeleteOldChatDebugRuns.
 //
 // finished_at is enforced as write-once at the SQL level: once
 // populated it cannot be overwritten by a later call. Callers
@@ -20595,6 +20629,22 @@ func (q *sqlQuerier) GetChatDebugLoggingAllowUsers(ctx context.Context) (bool, e
 	return allow_users, err
 }
 
+const getChatDebugRetentionDays = `-- name: GetChatDebugRetentionDays :one
+SELECT COALESCE(
+    (SELECT value::integer FROM site_configs
+     WHERE key = 'agents_chat_debug_retention_days'),
+    $1::integer
+) :: integer AS debug_retention_days
+`
+
+// Chat debug run retention window in days. 0 disables.
+func (q *sqlQuerier) GetChatDebugRetentionDays(ctx context.Context, defaultDebugRetentionDays int32) (int32, error) {
+	row := q.db.QueryRowContext(ctx, getChatDebugRetentionDays, defaultDebugRetentionDays)
+	var debug_retention_days int32
+	err := row.Scan(&debug_retention_days)
+	return debug_retention_days, err
+}
+
 const getChatDesktopEnabled = `-- name: GetChatDesktopEnabled :one
 SELECT
 	COALESCE((SELECT value = 'true' FROM site_configs WHERE key = 'agents_desktop_enabled'), false) :: boolean AS enable_desktop
@@ -21027,6 +21077,18 @@ func (q *sqlQuerier) UpsertChatDebugLoggingAllowUsers(ctx context.Context, allow
 	return err
 }
 
+const upsertChatDebugRetentionDays = `-- name: UpsertChatDebugRetentionDays :exec
+INSERT INTO site_configs (key, value)
+VALUES ('agents_chat_debug_retention_days', CAST($1 AS integer)::text)
+ON CONFLICT (key) DO UPDATE SET value = CAST($1 AS integer)::text
+WHERE site_configs.key = 'agents_chat_debug_retention_days'
+`
+
+func (q *sqlQuerier) UpsertChatDebugRetentionDays(ctx context.Context, debugRetentionDays int32) error {
+	_, err := q.db.ExecContext(ctx, upsertChatDebugRetentionDays, debugRetentionDays)
+	return err
+}
+
 const upsertChatDesktopEnabled = `-- name: UpsertChatDesktopEnabled :exec
 INSERT INTO site_configs (key, value)
 VALUES (