Document boolean and numeric CSV value typing

The preceding commits taught the import worker to coerce custom CSV
column values into JSON booleans and numbers via `coerceCustomValue`
in `apps/api/src/jobs/import-processor.ts`. Without a corresponding
docs update, users can't predict whether a cell like `01234` lands as
a string or as the number `1234`, or which segment-filter operators a
field will expose after import.

Add two bullets to the existing "Rules and limits" list in the
contact-import guide, adjacent to the **Date columns** bullet that
already documents value typing for ISO 8601 dates. The bullets mirror
that style and brevity: one names the boolean keyword set and the
toggle it unlocks in segment filters, the other names the numeric
pattern, the `gt`/`lt` operators it unlocks, and the deliberately
preserved-as-string forms (leading zeros, `+`-prefixed, scientific
notation) so users keep their IDs, zip codes, and phone numbers
intact.

No other content is touched. Closes the documentation gap for
useplunk/plunk#390.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/wiki/content/docs/guides/importing-contacts.mdx

@@ -37,6 +37,8 @@ In this example, every imported contact ends up with `data.firstName`, `data.pla
 - **Email column**: must be present and valid. Rows with missing or invalid emails are reported back as errors.
 - **Reserved column names**: `id`, `subscribed`, `createdAt`, `updatedAt`, and the auto-generated URL variables (`unsubscribeUrl`, etc.) are silently filtered out. Don't include them as columns.
 - **Date columns**: use ISO 8601 (`2026-05-06T12:00:00Z`) so they're typed as dates and become usable with `within` / `olderThan` segment operators.
+- **Boolean columns**: `true`, `false`, `yes`, `no` (case-insensitive) are stored as booleans and get the boolean toggle in segment filters.
+- **Numeric columns**: plain integers and decimals (`42`, `3.14`) are stored as numbers and become usable with `gt` / `lt` segment operators. Leading-zero values (`01234`), `+`-prefixed numbers, and scientific notation stay strings so IDs, zip codes, and phone numbers aren't corrupted.
 - **Existing contacts**: if a row's email matches an existing contact, the import **updates** the contact (merging the CSV's columns into `data`). It doesn't create a duplicate or overwrite the whole record.
 
 ## Importing your CSV