[benbrandt]

Every optional, non-patch v2 field now says how null is handled, using one standard wording: "Omitted and null are equivalent and mean …".

• Adds the statement to the 36 fields that had none. Examples: ListSessionsRequest.cwd/cursor, SessionInfo.title/updatedAt, config option description/category, Command.input, Usage token counts, content annotations/mimeType, ResourceLink and Icon fields, Annotations, and Error.data.
• Rewrites the existing variants ("Omitted or null both mean", "Omitted or null means") into the same wording.
• Overview (stable and draft) gets a general rule: omitted and null are equivalent for optional fields unless the field says otherwise, and upsert updates are the exception, where null clears.

Skipped on purpose: nextCursor (covered in #2567), the flattened idle stopReason (its struct doc already covers it), and JSON-RPC params. No CI lint. v2 only.