skills(objectstack-data): the tree rows state the landed reference contract (#16187)

`refuseForeignTreeReference` (packages/spec/src/data/object.zod.ts) makes a
`tree` field's `reference` optional and, when written, equal to the declaring
object's own name — the `extend` target on an extension. Any other object is
refused at parse with one issue at `fields.<field>.reference`. The published
skill still listed `reference` as a bare key prop, so a client agent reading it
had no way to know either half of the rule.

Two rows, no new section:

- `rules/field-types.md` — the Relational table's `tree` row now states
  optional-and-self-only and that a foreign object is refused at parse.
- `rules/relationships.md` — the `tree` row's bare "Optional" cell now says
  what is optional and contrasts the refusal with the `master_detail` row's
  lint-warned cell in the same column.

The token ratchet left `field-types.md` 27 bytes under its ceiling, so the row
is paid for inside the same file: the Media section's "no per-field attachment
config" note restated the top-of-file blockquote, which already refuses
`fileAttachmentConfig` by name, above every table. That file now shrinks.

The `Hierarchical: tree (Self-Reference)` example is unchanged — it already
writes the object's own name (`reference: 'category'` on `category`).


Claude-Session: https://claude.ai/code/session_019RfFHiRCSs3JXLK4cwcfox

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]

skills/objectstack-data/rules/field-types.md

@@ -88,7 +88,7 @@ options: [
 |:-----|:------------|:-----------|
 | `lookup` | Reference another object (independent) | `reference`, `lookupFilters`, `multiple`, `deleteBehavior` |
 | `master_detail` | Parent–child with lifecycle control | `reference`, `deleteBehavior` (`cascade`/`restrict` — `set_null` is refused) |
-| `tree` | Hierarchical self-reference | `reference` |
+| `tree` | Hierarchical self-reference | `reference` — optional; when written, this object's own name (any other object is refused at parse) |
 | `user` | Person picker — a lookup specialized to `sys_user` (assignee, watchers). Stored identically to `lookup` | `multiple` (collaborators), `defaultValue: 'current_user'` |
 
 > **`multiple: true` lookup ≠ junction object.** A multi-value lookup is stored
@@ -106,9 +106,6 @@ options: [
 | `video` | Video files | — |
 | `audio` | Audio files | — |
 
-There is no per-field attachment config (size limits, allowed types, storage) —
-storage concerns live outside the field schema.
-
 ## Embedded (JSON sub-objects)
 
 Stored as JSON on the parent row — no separate table / FK:

skills/objectstack-data/rules/relationships.md

@@ -8,7 +8,7 @@ Guide for modeling relationships between objects using `lookup`, `master_detail`
 |:-----|:----------|:---------|:--------|:---------|:---------|
 | `lookup` | Independent | Optional by default | Independent | Supported via `summary` | "Related to" |
 | `master_detail` | Coupled (cascade delete) | Forced only under `controlled_by_parent`; else lint-warned | Inherits parent | Supported via `summary` | "Owned by" |
-| `tree` | Self-reference | Optional | N/A | Not available | Hierarchical |
+| `tree` | Self-reference | Optional — and so is `reference`, which when written must name this object itself; any other object is refused at parse, not lint-warned | N/A | Not available | Hierarchical |
 
 ## When to Use lookup vs master_detail