/fix planner can now plan quarter comparison

src/config/prompts/planner.md

@@ -85,12 +85,20 @@ only a `TaskList` object that conforms to the provided schema.
     is still the ELEMENT type — a list of names is `"string"`, a date range is
     `"date"`. It is **never** `"list"`.
 - **Filters are ANDed — never stack two disjoint ranges on one column.** Every
-  `filters[]` entry combines with AND, so two non-overlapping `between` ranges on
-  the SAME column (e.g. `order_date` in Q1 2025 AND in Q1 2026) match **zero rows**.
-  For a multi-period comparison, either (a) use ONE spanning range
-  (`between ["2025-01-01", "2026-03-31"]`) and let the analysis step group by the
-  period, or (b) emit a SEPARATE `retrieve_data` task per period. Never AND two
-  ranges on one column expecting an OR.
+  `filters[]` entry combines with AND, so two non-overlapping `between` ranges on the
+  SAME column (e.g. a date in Q1 2025 AND in Q1 2026) match **zero rows**. Never AND
+  two ranges on one column expecting an OR.
+- **Period-over-period comparison = one query per period.** To compare two time
+  windows (e.g. Q1 2025 vs Q1 2026), emit a SEPARATE `retrieve_data` (+ its own
+  `analyze_*`) task for EACH period, each carrying its own `between` filter for that
+  window; the final answer compares the per-period results. Do NOT attempt it in one
+  query — there is no way to bucket a date into a quarter/month (no derived columns;
+  `group_by` is over EXISTING columns only), so a single spanning range collapses the
+  periods into one number and the comparison is lost.
+- **Time-bound a metric by the event's OWN date.** Filter a measure by the column
+  that dates the event you are measuring — revenue/sales by the order/transaction
+  date, NOT by an entity-creation date like `products.created_at` or
+  `customers.registered_at`. Join to the table that holds that date if needed.
 - **Use only the `args` a tool lists** — e.g. `analyze_aggregate` takes only
   `data`/`aggregations`/`group_by`, so never add `order_by`/`limit` to it.
 - **Top-N ("top/most/least N by <metric>") is a single `retrieve_data` query**,