docs(protocol): correct the two wrong defaultValue samples in schema.mdx (#7244) (#7376)

Both samples on `content/docs/protocol/objectql/schema.mdx` taught a shape that
today produces a rejected write or a bogus physical column DEFAULT. Measured on
`origin/main` before and after:

  * `due_datetime` carried a BARE CEL source string. `applyFieldDefaults`
    recognises an expression only by `{ dialect, source }`, so the bare string
    fell to the literal branch; an insert was refused
    ("bad_dt must be a valid datetime (ISO-8601)"), and the SQL DDL emitted
    `` `cur_due_datetime` datetime default 'daysFromNow(7)' `` -- the source
    text as a physical column DEFAULT. Replaced with the explicit envelope,
    which evaluates (2026-08-17T00:00:00.000Z from a 2026-08-10 now) and emits
    no column DEFAULT.

  * `revenue` carried `{ value: 0, currency: 'USD' }`. `currency` is in
    `NUMERIC_VALUE_TYPES`, so the stored contract is `z.number().finite()`;
    the object was refused by the record validator and dropped entirely by the
    SQL DDL (no default at all). Replaced with `0`, which stores and emits
    `float default '0'`.

The inline comments now state what the corrected samples actually mean,
including that `defaultValue` -- unlike a formula field's `expression` -- has
no bare-string CEL shorthand, which is the trap the old sample set.

Docs-only: no behaviour change, no changeset.


Claude-Session: https://claude.ai/code/session_01KJATVrh6V2ysutYUJigh3B

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

Author: os-help

content/docs/protocol/objectql/schema.mdx

@@ -382,7 +382,7 @@ revenue:
   label: Annual Revenue
   scale: 2
   precision: 18
-  defaultValue: { value: 0, currency: 'USD' }
+  defaultValue: 0   # currency stores a bare number — never { value, currency }
 ```
 
 #### Date/Time Fields
@@ -396,7 +396,10 @@ start_date:
 due_datetime:
   type: datetime
   label: Due Date & Time
-  defaultValue: "daysFromNow(7)"  # 7 days from now (CEL)
+  # A CEL default needs the explicit `{ dialect, source }` envelope. Unlike a
+  # formula field's `expression`, `defaultValue` has NO bare-string shorthand —
+  # a bare string is stored as that literal text.
+  defaultValue: { dialect: 'cel', source: 'daysFromNow(7)' }  # 7 days from now, at UTC midnight
 ```
 
 #### Boolean Fields