chore(conductor): Add new track 'Fix RRULE UNTIL conversion'

conductor/tracks/fix_rrule_until_conversion_20260219/spec.md

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+# Specification: Fix RRULE UNTIL Object Conversion
+
+## Overview
+The conversion of RRULE objects to strings fails for nested objects like `UNTIL` dates, resulting in `UNTIL=[object Object]`. This causes the `rrule` library to fail during expansion.
+
+## Functional Requirements
+- **Robust Value Formatting:** Implement `formatRRuleValue` to handle various types of RRULE values:
+    - **Date Objects:** Convert to `YYYYMMDDTHHMMSSZ`.
+    - **Nested Date Objects:** (e.g., `{ date: Date }` from `ts-ics`) Extract and convert.
+    - **Other Types:** Default to string conversion.
+- **Mapping Preservation:** Maintain the existing `RRULE_KEY_MAP` for key translation.
+
+## Implementation Steps
+1.  **Reproduce:** Add test `expandRecurrences - object rrule with until` in `icalendar_test.ts` using a Date object for `until`. Verify it fails with `Invalid UNTIL value`.
+2.  **Fix:** Update `expandRecurrences` in `icalendar.ts` to use a formatting helper for values.
+3.  **Verify:** Confirm the test case passes.
+
+## Acceptance Criteria
+- [ ] Test `expandRecurrences - object rrule with until` passes.
+- [ ] The generated string correctly represents the date (e.g., `UNTIL=20260219T000000Z`).