feat(field): lastModifiedTime expression field references

Parse LAST_MODIFIED_TIME([Name]/{fldId}) references into trackedFieldIds so
the modification time only resets when a referenced field changes.

- core: relax expression from z.literal('LAST_MODIFIED_TIME()') to z.string(),
  add parseLastModifiedTimeReferences() + spec
- v1 field-supplement.service: resolve trackedFieldIds from expression on
  create/convert (incl. same-type merge), 400 on unknown field name
- v2 field-open-api-v2.service: resolve trackedFieldIds from expression on
  create/createFields/convert
- docs: field-types.md tracked-field reference section

.agents/skills/inkreach-teable/api-reference/field-types.md

@@ -320,6 +320,42 @@ inkreach-teable field create --table-id tblXXX --name "Delete" --type button \
 
 ---
 
+### Last Modified Time (tracked-field references)
+
+A `lastModifiedTime` field can track **only specific fields** instead of all of them, so
+the timestamp resets when a referenced field changes but stays put on ordinary edits.
+Write the references inside the `expression` option:
+
+```bash
+# Track by field name — resets only when 流转状态 or 流转角色 change
+inkreach-teable field create --table-id tblXXX --name "流转时间" --type lastModifiedTime \
+  --options '{"expression":"LAST_MODIFIED_TIME([流转状态],[流转角色])"}'
+
+# Track by field id (mixed syntax also allowed)
+inkreach-teable field create --table-id tblXXX --name "流转时间" --type lastModifiedTime \
+  --options '{"expression":"LAST_MODIFIED_TIME({fldAbc123},{fldDef456})"}'
+```
+
+| Reference syntax | Meaning |
+|---|---|
+| `LAST_MODIFIED_TIME()` | Track **all** fields (default) |
+| `LAST_MODIFIED_TIME([Name])` | Reference a field by its name |
+| `LAST_MODIFIED_TIME({fldId})` | Reference a field by its id |
+
+**Key behaviors**:
+
+1. The expression is parsed server-side into `trackedFieldIds` (the field's effective
+   `options.trackedFieldIds`); the stored `expression` is normalized back to the canonical
+   `LAST_MODIFIED_TIME()` form while `trackedFieldIds` carries the references.
+2. An unresolvable field **name** returns `400 Unknown field referenced in expression: <name>`;
+   unresolvable `{fldId}` values are simply ignored (harmless — never match).
+3. You can also set `options.trackedFieldIds` directly (an array of field ids); an explicit
+   `trackedFieldIds` wins over the expression when both are present.
+4. Works on create **and** convert (`PUT /api/table/:tableId/field/:fieldId/convert` with
+   `{"type":"lastModifiedTime","options":{"expression":"..."}}`).
+
+---
+
 ## Smart Inference
 
 ### Type inference from `--name`